Video: convert sprite pixels to display format

remove the old (unused) ConvertToVideoFormat method in Video and have the CreateSprite method automatically convert the sprite
GetScreenshot: create the surface sprite directly

avoid calling createsprite so we dont do unnecessary pixel conversion
SwapBuffers: generalize cursor/tooltip drawing

should re-enable both on SDL2 dirver.
Video: move sprite mirroring logic to base
SDLVideo: update for SDL2 api change

the unicode field is removed from the keysym struct