Initial SDL2 renderer impmlementation work.
Stub out unfinished implementations
i_video.c: remove old cruft and dead code
remove hi-res filters
sdl2: remove unsupported platform-specific folders
sdl2: fix memory detection on 64-bit linux
sdl2: it works
sdl2: restore keyboard input
sdl2: support all windowed mode resolutions
sdl2: support windowed fullscreen as default
sdl2: aspect correction in fullscreen, clear on blit
sdl2: fix mode changing
sdl2: Fix changing between fullscreen/windowed
sdl2: Make -win toggle cvar instead of disabling fullscreen
sdl2: Zero out buffer on sound callback
sdl2: Bump up mixer support to SDL2_mixxer
sdl2: Restore mouse motion support
sdl2: Fix mouse button input
Win32 CPU cpuinfo was also using SDL_MMXExt() and SDL_Had3DNewExt()
disabled DEBUG section because of use of SDL_VideoDriverName()