sdl2: enable recording of frames and audio into a video format

[deltabeard]

I think this will be better for making animations and videos than dumping every single frame in an uncompressed bitmap.

Requirements:

• Video and audio in lossless format if only one format enabled for each.
• Compile time option to enable or disable this bulky feature.
• Start/Stop video record keyboard shortcut.

Optional:

• Recording at correct frame rate.
• Ability to pipe video and audio to an external program like ffmpeg, instead of using an encoder directly.