stb_image_write.h: Fix compilation in C++11 mode

clang says:
error: non-constant-expression cannot be narrowed from type 'int'
      to 'unsigned char' in initializer list [-Wc++11-narrowing]

so I explicitly cast affected stuff to unsigned char.

stb_image_write.h

@@ -1283,7 +1283,8 @@ static int stbi_write_jpg_core(stbi__write_context *s, int width, int height, in
    {
       static const unsigned char head0[] = { 0xFF,0xD8,0xFF,0xE0,0,0x10,'J','F','I','F',0,1,1,0,0,1,0,1,0,0,0xFF,0xDB,0,0x84,0 };
       static const unsigned char head2[] = { 0xFF,0xDA,0,0xC,3,1,0,2,0x11,3,0x11,0,0x3F,0 };
-      const unsigned char head1[] = { 0xFF,0xC0,0,0x11,8,height>>8,height&0xFF,width>>8,width&0xFF,3,1,0x11,0,2,0x11,1,3,0x11,1,0xFF,0xC4,0x01,0xA2,0 };
+      const unsigned char head1[] = { 0xFF,0xC0,0,0x11,8,(unsigned char)(height>>8),STBIW_UCHAR(height),(unsigned char)(width>>8),STBIW_UCHAR(width),
+                                      3,1,0x11,0,2,0x11,1,3,0x11,1,0xFF,0xC4,0x01,0xA2,0 };
       s->func(s->context, (void*)head0, sizeof(head0));
       s->func(s->context, (void*)YTable, sizeof(YTable));
       stbiw__putc(s, 1);