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.)

jdmerge.h

@@ -4,13 +4,13 @@
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1994-1996, Thomas G. Lane.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2020, D. R. Commander.
+ * Copyright (C) 2020, 2022, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  */
 
 #define JPEG_INTERNALS
-#include "jpeglib.h"
+#include "jpeglibint.h"
 
 #ifdef UPSAMPLE_MERGING_SUPPORTED