SDLRenderDevice fixes

These are some things I found while working on SDL2 support that also applied to SDL1.
* 2e087d4 removes the SDL_DisplayFormatAlpha() call when copying Sprite objects.
* 36043d8 removes RenderDevice::setAlpha() and RenderDevice::setColorKey(), which had no effect in their previous usage.
* 10a2eee fixes a crash from freeing an SDL_Surface twice