Add video playback

[grantramsay]

Works although:

• video looks a bit corrupted
• aspect ratio is lost
• a bit complicated/hacky, probably a much cleaner way to do it

Details from source:

// Adapted from https://wiki.videolan.org/LibVLC_SampleCode_SDL/
// Make sure VLC_PLUGIN_PATH env variable is set and using newish VLC (tested with 3.0.4).
// Currently a pretty hacky implementation, and there's bound to be a much easier way.
// Creates a new SDL window and GL context for video, as otherwise SDL_CreateRenderer alters
// the current GL context, causing a crash. Attempts to swap GL context back and forth when
// video is playing/stopped.
// VIDEO_DIRECT_WINDOW_RENDER is an attempt at rendering directly to an existing window,
// although seems to crash after video has finished. Also seems to require all the functions
// in sdl_gl_funcs.h/.cpp to be renamed to non-default GL names or crashes beforehand.

Fix for #27

Seemed like an interesting task so I had a look. Might be worth just leaving this PR open and someone with more OpenGL/VLC/SDL knowledge could fix it up... would be a good starting point at least.

[grantramsay]

Reckon we ditch vlc in favour of libmpv:

• also cross platform
• SMK playback not corrupted (maybe an issue in vlc codec?)
• better integration with openGL (most bad stuff in this PR was because of this)
• can run in same SDL window as other rendering and keeps the correct video aspect ratio Working on it now, will close this one