Remove unused SDL2 surface in compat code

mupen64plus doesn't support 2D video output because it forces OPENGL mode.
Unfortunately, the rest of the compat code still assumes that surfaces for 2D
data are used and overwrites the data when the window is changed.

The surface code can be removed to avoid these problems.
Close SDL2 window when quitting Video subsystem

SDL2 doesn't close windows anymore when the video subsystem is closed.
Therefore, the window created by the SDL2 compat layer has to be closed
manually to avoid orphan windows in frontends like m64py in non-VidExt mode.
Merge pull request #7 from ecsv/sdl2_compat

Cleanup SDL2 compat code to avoid problems in non-VidExt m64py