Fixed #119.
Initial modular renderer commit. Breaks Mac + Linux + Cmake builds. Please wait for future commits =]
Works on Windows with: client (modular rd), listen server (modular rd), dedicated server (static rd)
Fixed error in PNG_Pack that caused PNG_Save to fail
Added timestamps to screenshot filenames. Added screenshot_png. Fixed r_aviMotionJpegQuality and r_screenshotJpegQuality not being initialised
Adjust some cvar defaults and limits as noted in #70
Attempt to synchronise snapshots/sec with server framerate