Changes relevant to mod developers (SDK):
- Added UploadImage to idRenderSystem interface. This lets the user blit images to the renderer.
- Supports fs_game_base; this lets you base a mod off base game + d3xp + your own content.
- Most of the download redirection is handled in the game code, and can be extended.
Linux specific:
- ALSA device opened non-blocking to avoid hangs.

