Support 8-bit & 12-bit JPEGs using the same build

Partially implements #199

This commit also implements a request from #178 (the ability to compile
the libjpeg example as a standalone program.)

jpeglib.h

@@ -5,7 +5,7 @@
  * Copyright (C) 1991-1998, Thomas G. Lane.
  * Modified 2002-2009 by Guido Vollbeding.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2009-2011, 2013-2014, 2016-2017, 2020, D. R. Commander.
+ * Copyright (C) 2009-2011, 2013-2014, 2016-2017, 2020, 2022, D. R. Commander.
  * Copyright (C) 2015, Google, Inc.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
@@ -70,6 +70,8 @@ typedef JSAMPLE *JSAMPROW;      /* ptr to one image row of pixel samples. */
 typedef JSAMPROW *JSAMPARRAY;   /* ptr to some rows (a 2-D sample array) */
 typedef JSAMPARRAY *JSAMPIMAGE; /* a 3-D sample array: top index is color */
 
+#ifndef JPEG12LIB_H
+
 typedef JCOEF JBLOCK[DCTSIZE2]; /* one block of coefficients */
 typedef JBLOCK *JBLOCKROW;      /* pointer to one row of coefficient blocks */
 typedef JBLOCKROW *JBLOCKARRAY;         /* a 2-D array of coefficient blocks */
@@ -264,6 +266,8 @@ typedef enum {
   JDITHER_FS              /* Floyd-Steinberg error diffusion dither */
 } J_DITHER_MODE;
 
+#endif /* JPEG12LIB_H */
+
 
 /* Common fields between JPEG compression and decompression master structs. */
 
@@ -822,6 +826,8 @@ struct jpeg_source_mgr {
  * successful.
  */
 
+#ifndef JPEG12LIB_H
+
 #define JPOOL_PERMANENT  0      /* lasts until master record is destroyed */
 #define JPOOL_IMAGE      1      /* lasts until done with image/datastream */
 #define JPOOL_NUMPOOLS   2
@@ -829,6 +835,8 @@ struct jpeg_source_mgr {
 typedef struct jvirt_sarray_control *jvirt_sarray_ptr;
 typedef struct jvirt_barray_control *jvirt_barray_ptr;
 
+#endif
+
 
 struct jpeg_memory_mgr {
   /* Method pointers */