Remove SDL 1.2 and Cg support

We now uniformly use SDL2 >= 2.0.2 and Gl everywhere.  This is confirmed to fix #4945, #4580 and #5228, and it may also fix other issues like #2430, #5156, #5157 (these need testing).

This will allow for a bunch of future improvements such as proper scroll physics in OSX, copy/paste in text fields, #3774, #4540, etc.