Use SDL_PremultiplySurfaceAlpha() when creating NSImages

src/video/cocoa/SDL_cocoavideo.m

@@ -253,7 +253,6 @@ NSImage *Cocoa_CreateImage(SDL_Surface *surface)
     SDL_Surface *converted;
     NSBitmapImageRep *imgrep;
     Uint8 *pixels;
-    int i;
     NSImage *img;
 
     converted = SDL_ConvertSurface(surface, SDL_PIXELFORMAT_RGBA32);
@@ -261,6 +260,9 @@ NSImage *Cocoa_CreateImage(SDL_Surface *surface)
         return nil;
     }
 
+    /* Premultiply the alpha channel */
+    SDL_PremultiplySurfaceAlpha(converted, SDL_FALSE);
+
     imgrep = [[NSBitmapImageRep alloc] initWithBitmapDataPlanes:NULL
                                                      pixelsWide:converted->w
                                                      pixelsHigh:converted->h
@@ -281,15 +283,6 @@ NSImage *Cocoa_CreateImage(SDL_Surface *surface)
     SDL_memcpy(pixels, converted->pixels, (size_t)converted->h * converted->pitch);
     SDL_DestroySurface(converted);
 
-    /* Premultiply the alpha channel */
-    for (i = (surface->h * surface->w); i--;) {
-        Uint8 alpha = pixels[3];
-        pixels[0] = (Uint8)(((Uint16)pixels[0] * alpha) / 255);
-        pixels[1] = (Uint8)(((Uint16)pixels[1] * alpha) / 255);
-        pixels[2] = (Uint8)(((Uint16)pixels[2] * alpha) / 255);
-        pixels += 4;
-    }
-
     img = [[NSImage alloc] initWithSize:NSMakeSize(surface->w, surface->h)];
     if (img != nil) {
         [img addRepresentation:imgrep];