Add SDL_RenderSetVSync()

Currently, if an application wants to toggle VSync, they'd have to tear
down the renderer and recreate it. This patch fixes that by letting
applications call SDL_RenderSetVSync().

This is the same as the patch in #3673, except it applies to all
renderers (including PSP, even thought it seems that the VSync flag is
disabled for that renderer). Furthermore, the renderer flags also change
as well, which #3673 didn't do. It is also an API instead of using hint
callbacks (which could be potentially dangerous).

Closes #3673.

src/render/SDL_sysrender.h

@@ -163,6 +163,8 @@ struct SDL_Renderer
 
     void (*DestroyRenderer) (SDL_Renderer * renderer);
 
+    int (*SetVSync) (SDL_Renderer * renderer, int vsync);
+
     int (*GL_BindTexture) (SDL_Renderer * renderer, SDL_Texture *texture, float *texw, float *texh);
     int (*GL_UnbindTexture) (SDL_Renderer * renderer, SDL_Texture *texture);