Refactor code in wxQuantize() for MSVC to avoid crash

VC++ compilers (at least up to VS 2015) seem to generate incorrect code for the dithering code taken from libjpeg (pass2_fs_dither() function), where it is known to create problems too.
Refactor the code to avoid invalid optimization in this function and turn optimization on.

Closes #17764.

docs/changes.txt

@@ -119,6 +119,7 @@ All (GUI):
 - Add wxDataViewRenderer::GetAccessibleDescription().
 - Improve wxImage::Scale() handling of pixels with alpha channel (Tim Kosse).
 - Fix parsing of RGBA strings in wxColour (Laurent Poujoulat).
+- Refactor code in wxQuantize() for MSVC to avoid crash.
 
 wxGTK:
 

src/common/quantize.cpp

@@ -1026,17 +1026,6 @@ pass2_no_dither (j_decompress_ptr cinfo,
   }
 }
 
-#ifdef __VISUALC__
-/*
-  for (yet) unknown reasons, the following code produces
-  bad results or even crashes if the code optimization is
-  enabled in VC++
-
-  disable optimization for now until the reason is discovered
-*/
-  #pragma optimize ("", off)
-#endif
-
 void
 pass2_fs_dither (j_decompress_ptr cinfo,
          JSAMPARRAY input_buf, JSAMPARRAY output_buf, int num_rows)
@@ -1137,7 +1126,18 @@ pass2_fs_dither (j_decompress_ptr cinfo,
 
     bnexterr = cur0;    /* Process component 0 */
     delta = cur0 * 2;
+/*
+    Calculating cur0, cur1, cur2 gives bad results if code
+    optimization is enabled in VC++. This is apparently caused
+    by some bug in the compiler.
+    Use refactored code until it is fixed.
+*/
+#ifdef __VISUALC__
+    /* Use refactored code to prevent invalid optimization. */
+    cur0 *= 3;          /* form error * 3 */
+#else
     cur0 += delta;      /* form error * 3 */
+#endif
     errorptr[0] = (FSERROR) (bpreverr0 + cur0);
     cur0 += delta;      /* form error * 5 */
     bpreverr0 = belowerr0 + cur0;
@@ -1145,7 +1145,12 @@ pass2_fs_dither (j_decompress_ptr cinfo,
     cur0 += delta;      /* form error * 7 */
     bnexterr = cur1;    /* Process component 1 */
     delta = cur1 * 2;
+#ifdef __VISUALC__
+    /* Use refactored code to prevent invalid optimization. */
+    cur1 *= 3;          /* form error * 3 */
+#else
     cur1 += delta;      /* form error * 3 */
+#endif
     errorptr[1] = (FSERROR) (bpreverr1 + cur1);
     cur1 += delta;      /* form error * 5 */
     bpreverr1 = belowerr1 + cur1;
@@ -1153,7 +1158,12 @@ pass2_fs_dither (j_decompress_ptr cinfo,
     cur1 += delta;      /* form error * 7 */
     bnexterr = cur2;    /* Process component 2 */
     delta = cur2 * 2;
+#ifdef __VISUALC__
+    /* Use refactored code to prevent invalid optimization. */
+    cur2 *= 3;          /* form error * 3 */
+#else
     cur2 += delta;      /* form error * 3 */
+#endif
     errorptr[2] = (FSERROR) (bpreverr2 + cur2);
     cur2 += delta;      /* form error * 5 */
     bpreverr2 = belowerr2 + cur2;
@@ -1177,9 +1187,6 @@ pass2_fs_dither (j_decompress_ptr cinfo,
     errorptr[2] = (FSERROR) bpreverr2;
   }
 }
-#ifdef __VISUALC__
-  #pragma optimize ("", on)
-#endif
 
 /*
  * Initialize the error-limiting transfer function (lookup table).