Enhancements for SDL_PremultiplyAlpha()

The function can now convert between pixels of different formats, and takes a parameter to control whether the premultiplication is done in sRGB or linear space.

Also added SDL_PremultiplySurfaceAlpha(), which can premultiply the pixels of a surface in-place.

src/dynapi/SDL_dynapi.sym

@@ -610,6 +610,7 @@ SDL3_0.0.0 {
     SDL_PollEvent;
     SDL_PostSemaphore;
     SDL_PremultiplyAlpha;
+    SDL_PremultiplySurfaceAlpha;
     SDL_PumpEvents;
     SDL_PushEvent;
     SDL_PutAudioStreamData;