examples/save-rendering-to-bitmaps: Dump alpha channel.

examples/renderer/20-blending triggered this: the renderer's framebuffer has
an alpha channel, even if the window itself doesn't, so we ended up with some
weird rendering in the onmouseover.webp when I rebuilt it.

examples/save-rendering-to-bitmaps.h

@@ -32,6 +32,11 @@ static bool SAVERENDERING_SDL_RenderPresent(SDL_Renderer *renderer)
     if (!surface) {
         SDL_LogError(SDL_LOG_CATEGORY_RENDER, "Failed to read pixels for frame #%u! (%s)", framenum, SDL_GetError());
     } else {
+        SDL_Surface *cvt = SDL_ConvertSurface(surface, SDL_PIXELFORMAT_RGBX32);
+        if (cvt) {
+            SDL_DestroySurface(surface);
+            surface = cvt;
+        }
         char fname[64];
         SDL_snprintf(fname, sizeof (fname), "frame%05u.png", framenum);
         if (!SDL_SavePNG(surface, fname)) {