cmake: added modules for sdl2, sdl2_mixer and glew

first two from Benjamin Eikel and co
last from imageutilities
cmake: added support for building all three versions of SDLVideo

pass -DUSE_SDL2 if you prefer SDL2 on installs with both
pass -DUSE_OPENGL=1 for regular OpenGL, 2 for GLES

tested only the default path, which is still SDL1