Save non-alpha pngs as RGB not RGBA

Author: ankith26

src/IMG_png.c

@@ -766,14 +766,41 @@ static int IMG_SavePNG_RW_miniz(SDL_Surface *surface, SDL_RWops *dst)
         return IMG_SetError("Passed NULL dst");
     }
 
-    if (surface->format->format == png_format) {
-        png = tdefl_write_image_to_png_file_in_memory(surface->pixels, surface->w, surface->h, surface->format->BytesPerPixel, surface->pitch, &size);
-    } else {
-        SDL_Surface *cvt = SDL_ConvertSurfaceFormat(surface, png_format, 0);
-        if (cvt) {
-            png = tdefl_write_image_to_png_file_in_memory(cvt->pixels, cvt->w, cvt->h, cvt->format->BytesPerPixel, cvt->pitch, &size);
-            SDL_FreeSurface(cvt);
+    // TODO:
+    // how to handle indexed+alpha images?
+    // how to handle colorkey?
+    Uint32 cvt_fmt = SDL_ISPIXELFORMAT_ALPHA(surface->format->format) ? png_format : SDL_PIXELFORMAT_RGB24;
+    SDL_Surface *cvt = NULL;
+    Uint8 *pixels = surface->pixels;
+    int bpp = surface->format->BytesPerPixel;
+    int pitch = surface->pitch;
+    if (surface->format->format != cvt_fmt) {
+        cvt = SDL_ConvertSurfaceFormat(surface, cvt_fmt, 0);
+        pixels = cvt ? cvt->pixels : NULL;
+        bpp = cvt ? cvt->format->BytesPerPixel : 0;
+        pitch = cvt ? cvt->pitch : 0;
+    }
+    if (pixels) {
+        int w = surface->w;
+        int h = surface->h;
+        int tightened_pitch = bpp * w;
+        if (pitch != tightened_pitch) {
+            /* There is extra padding that we need to get rid of */
+            Uint8 *tightened_pixels = SDL_malloc(tightened_pitch * h);
+            if (tightened_pixels) {
+                for (int y = 0; y < h; y++) {
+                    memcpy(tightened_pixels + y * tightened_pitch, pixels + y * pitch, tightened_pitch);
+                }
+                png = tdefl_write_image_to_png_file_in_memory(tightened_pixels, w, h, bpp, tightened_pitch, &size);
+                SDL_free(tightened_pixels);
+            }
         }
+        else {
+            png = tdefl_write_image_to_png_file_in_memory(pixels, w, h, bpp, pitch, &size);
+        }
+    }
+    if (cvt) {
+        SDL_FreeSurface(cvt);
     }
     if (png) {
         if (SDL_RWwrite(dst, png, size, 1)) {