TurboJPEG C API: Add BMP/PPM load/save functions

The main justification for this is to provide new libjpeg-turbo users
with a quick & easy way of developing a complete JPEG
compression/decompression program without requiring them to build
libjpeg-turbo from source (which was necessary in order to use the
project-private bmp API) or to use external libraries.  These new
functions build upon significant enhancements to rdbmp.c, wrbmp.c,
rdppm.c, and wrppm.c which allow those engines to convert directly
between the native pixel format of the file and a pixel format
("colorspace" in libjpeg parlance) specified by the calling program.
rdbmp.c and wrbmp.c have also been modified such that the calling
program can choose to read or write image rows in the native (bottom-up)
order of the file format, thus eliminating the need to use an inversion
array.  tjLoadImage() and tjSaveImage() leverage these new underlying
features in order to significantly improve upon the performance of the
old bmp API.

Because these new functions cannot work without the libjpeg-turbo
colorspace extensions, the libjpeg-compatible code in turbojpeg.c has
been removed.  That code was only there to serve as an example of how
to use the TurboJPEG API on top of libjpeg, but more specific, buildable
examples now exist in the https://github.com/libjpeg-turbo/ijg
repository.

turbojpeg.h

@@ -249,7 +249,11 @@ enum TJPF
    * CMYK pixels into a YCCK JPEG image (see #TJCS_YCCK) and decompressing YCCK
    * JPEG images into CMYK pixels.
    */
-  TJPF_CMYK
+  TJPF_CMYK,
+  /**
+   * Unknown pixel format.  Currently this is only used by #tjLoadImage().
+   */
+  TJPF_UNKNOWN = -1
 };
 
 
@@ -1514,6 +1518,86 @@ DLLEXPORT int DLLCALL tjDestroy(tjhandle handle);
 DLLEXPORT unsigned char* DLLCALL tjAlloc(int bytes);
 
 
+/**
+ * Load an uncompressed image from disk into memory.
+ *
+ * @param filename name of a file containing an uncompressed image in Windows
+ * BMP or PBMPLUS (PPM/PGM) format
+ *
+ * @param width pointer to an integer variable that will receive the width (in
+ * pixels) of the uncompressed image
+ *
+ * @param align row alignment of the image buffer to be returned (must be a
+ * power of 2.)  For instance, setting this parameter to 4 will cause all rows
+ * in the image buffer to be padded to the nearest 32-bit boundary, and setting
+ * this parameter to 1 will cause all rows in the image buffer to be unpadded.
+ *
+ * @param height pointer to an integer variable that will receive the height
+ * (in pixels) of the uncompressed image
+ *
+ * @param pixelFormat pointer to an integer variable that specifies or will
+ * receive the pixel format of the uncompressed image buffer.  If
+ * <tt>*pixelFormat</tt> is set to @ref TJPF_UNKNOWN prior to calling this
+ * function, then the uncompressed image buffer returned by the function will
+ * use the most optimal pixel format for the file type, and
+ * <tt>*pixelFormat</tt> will contain the ID of this pixel format upon
+ * successful return from the function.  Otherwise, the uncompressed image
+ * buffer will use the @ref TJPF "pixel format" specified in
+ * <tt>*pixelFormat</tt>, and pixel format conversion will be performed if
+ * necessary.  If <tt>*pixelFormat</tt> is set to @ref TJPF_CMYK, then the RGB
+ * or grayscale pixels stored in the file will be converted using a quick &
+ * dirty algorithm that is suitable only for testing purposes (proper
+ * conversion between CMYK and other formats requires a color management
+ * system.)
+ *
+ * @param flags the bitwise OR of one or more of the @ref TJFLAG_BOTTOMUP
+ * "flags".
+ *
+ * @return a pointer to a newly-allocated buffer containing the uncompressed
+ * image, converted to the chosen pixel format and with the chosen row
+ * alignment, or NULL if an error occurred (see #tjGetErrorStr2().)  This
+ * buffer should be freed using #tjFree().
+ */
+DLLEXPORT unsigned char* DLLCALL tjLoadImage(const char *filename, int *width,
+  int align, int *height, int *pixelFormat, int flags);
+
+
+/**
+ * Save an uncompressed image from memory to disk.
+ *
+ * @param filename name of a file to which to save the uncompressed image.
+ * The image will be stored in Windows BMP or PBMPLUS (PPM/PGM) format,
+ * depending on the file extension.
+ *
+ * @param buffer pointer to an image buffer containing RGB, grayscale, or
+ * CMYK pixels to be saved
+ *
+ * @param width width (in pixels) of the uncompressed image
+ *
+ * @param pitch bytes per line in the image buffer.  Setting this parameter to
+ * 0 is the equivalent of setting it to
+ * <tt>width * #tjPixelSize[pixelFormat]</tt>.
+ *
+ * @param height height (in pixels) of the uncompressed image
+ *
+ * @param pixelFormat pixel format of the image buffer (see @ref TJPF
+ * "Pixel formats".)  If this parameter is set to @ref TJPF_GRAY, then the
+ * image will be stored in PGM or 8-bit (indexed color) BMP format.  Otherwise,
+ * the image will be stored in PPM or 24-bit BMP format.  If this parameter
+ * is set to @ref TJPF_CMYK, then the CMYK pixels will be converted to RGB
+ * using a quick & dirty algorithm that is suitable only for testing (proper
+ * conversion between CMYK and other formats requires a color management
+ * system.)
+ *
+ * @param flags the bitwise OR of one or more of the @ref TJFLAG_BOTTOMUP
+ * "flags".
+ *
+ * @return 0 if successful, or -1 if an error occurred (see #tjGetErrorStr2().)
+ */
+DLLEXPORT int DLLCALL tjSaveImage(const char *filename, unsigned char *buffer,
+  int width, int pitch, int height, int pixelFormat, int flags);
+
+
 /**
  * Free an image buffer previously allocated by TurboJPEG.  You should always
  * use this function to free JPEG destination buffer(s) that were automatically