stb_image: Reorder format test sequence

Put the formats that start with a clear magic number first,
the dodgy ones that don't have much of a distinctive header
should be tested for later after we've ruled out the clearer
ones.

Fixes issue #787, hopefully. (Never got a clean repro.)

stb_image.h

@@ -1117,9 +1117,8 @@ static void *stbi__load_main(stbi__context *s, int *x, int *y, int *comp, int re
    ri->channel_order = STBI_ORDER_RGB; // all current input & output are this, but this is here so we can add BGR order
    ri->num_channels = 0;
 
-   #ifndef STBI_NO_JPEG
-   if (stbi__jpeg_test(s)) return stbi__jpeg_load(s,x,y,comp,req_comp, ri);
-   #endif
+   // test the formats with a very explicit header first (at least a FOURCC
+   // or distinctive magic number first)
    #ifndef STBI_NO_PNG
    if (stbi__png_test(s))  return stbi__png_load(s,x,y,comp,req_comp, ri);
    #endif
@@ -1137,6 +1136,13 @@ static void *stbi__load_main(stbi__context *s, int *x, int *y, int *comp, int re
    #ifndef STBI_NO_PIC
    if (stbi__pic_test(s))  return stbi__pic_load(s,x,y,comp,req_comp, ri);
    #endif
+
+   // then the formats that can end up attempting to load with just 1 or 2
+   // bytes matching expectations; these are prone to false positives, so
+   // try them later
+   #ifndef STBI_NO_JPEG
+   if (stbi__jpeg_test(s)) return stbi__jpeg_load(s,x,y,comp,req_comp, ri);
+   #endif
    #ifndef STBI_NO_PNM
    if (stbi__pnm_test(s))  return stbi__pnm_load(s,x,y,comp,req_comp, ri);
    #endif