It turns out both the software and the OpenGL renderer had the same problem.

According to the spec, when SDL_SRCALPHA is set, the colorkey is ignored, so
we just need to convert the colorkey into the alpha channel.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%403248

src/video/SDL_surface.c

@@ -270,7 +270,7 @@ SDL_SetColorKey(SDL_Surface * surface, Uint32 flag, Uint32 key)
 }
 
 /* This is a fairly slow function to switch from colorkey to alpha */
-void
+static void
 SDL_ConvertColorkeyToAlpha(SDL_Surface * surface)
 {
     int x, y;
@@ -813,6 +813,7 @@ SDL_ConvertSurface(SDL_Surface * surface,
                     &keyG, &keyB, &keyA);
         SDL_SetColorKey(convert, 1,
                         SDL_MapRGBA(convert->format, keyR, keyG, keyB, keyA));
+        SDL_ConvertColorkeyToAlpha(convert);
     }
     convert->map->info.r = surface->map->info.r;
     convert->map->info.g = surface->map->info.g;

src/video/SDL_video.c

@@ -46,9 +46,6 @@
 #endif
 #endif /* SDL_VIDEO_OPENGL */
 
-/* From SDL_surface.c */
-extern void SDL_ConvertColorkeyToAlpha(SDL_Surface * surface);
-
 /* Available video drivers */
 static VideoBootStrap *bootstrap[] = {
 #if SDL_VIDEO_DRIVER_COCOA
@@ -1588,7 +1585,6 @@ SDL_CreateTextureFromSurface(Uint32 format, SDL_Surface * surface)
             }
             dst = SDL_ConvertSurface(surface, dst_fmt, 0);
             if (dst) {
-                SDL_ConvertColorkeyToAlpha(dst);
                 SDL_UpdateTexture(textureID, NULL, dst->pixels, dst->pitch);
                 SDL_FreeSurface(dst);
             }