Changes to get Ren'Py compiling with pygame_sdl2/sdl2.
RWOpsFromPythonThreaded -> RWOpsFromPython

In pygame_sdl2, rwops is always threadsafe.
Zero out the audio buffer before mixing samples into it.

This is necessary for SDL 2.
Report a traceback when pysdlsound fails to import.
nogil_copy should copy without using the alpha channel.
Merge branch 'master' into sdl2
Only copy ANGLE files if they're out of date.
Build w/ SDL2 on Windows.

For now, ANGLE is disabled. At some point, we'll try to use SDL to
initialize it.
We no longer need to force windib. (Which no longer exists.)
Merge branch 'master' into sdl2
Use the single-file pygame_sdl2 C api.
Initialize angle under pygame_sdl2.