stb_image_write: Support writing BMPs with alpha

Fixes issue #1159.

stb_image_write.h

@@ -491,11 +491,22 @@ static int stbiw__outfile(stbi__write_context *s, int rgb_dir, int vdir, int x,
 
 static int stbi_write_bmp_core(stbi__write_context *s, int x, int y, int comp, const void *data)
 {
-   int pad = (-x*3) & 3;
-   return stbiw__outfile(s,-1,-1,x,y,comp,1,(void *) data,0,pad,
-           "11 4 22 4" "4 44 22 444444",
-           'B', 'M', 14+40+(x*3+pad)*y, 0,0, 14+40,  // file header
-            40, x,y, 1,24, 0,0,0,0,0,0);             // bitmap header
+   if (comp != 4) {
+      // write RGB bitmap
+      int pad = (-x*3) & 3;
+      return stbiw__outfile(s,-1,-1,x,y,comp,1,(void *) data,0,pad,
+              "11 4 22 4" "4 44 22 444444",
+              'B', 'M', 14+40+(x*3+pad)*y, 0,0, 14+40,  // file header
+               40, x,y, 1,24, 0,0,0,0,0,0);             // bitmap header
+   } else {
+      // RGBA bitmaps need a v4 header
+      // use BI_BITFIELDS mode with 32bpp and alpha mask
+      // (straight BI_RGB with alpha mask doesn't work in most readers)
+      return stbiw__outfile(s,-1,-1,x,y,comp,1,(void *)data,1,0,
+         "11 4 22 4" "4 44 22 444444 4444 4 444 444 444 444",
+         'B', 'M', 14+108+x*y*4, 0, 0, 14+108, // file header
+         108, x,y, 1,32, 3,0,0,0,0,0, 0xff0000,0xff00,0xff,0xff000000u, 0, 0,0,0, 0,0,0, 0,0,0, 0,0,0); // bitmap V4 header
+   }
 }
 
 STBIWDEF int stbi_write_bmp_to_func(stbi_write_func *func, void *context, int x, int y, int comp, const void *data)
@@ -1619,6 +1630,7 @@ STBIWDEF int stbi_write_jpg(char const *filename, int x, int y, int comp, const
 /* Revision history
       1.15  (          )
              make Deflate code emit uncompressed blocks when it would otherwise expand
+             support writing BMPs with alpha channel
       1.14  (2020-02-02) updated JPEG writer to downsample chroma channels
       1.13
       1.12