Allow dynamic frame sizes for FFmpeg inputs.

Allow dynamic frame sizes for FFmpeg inputs.

If we need a larger frame size than FRAME_SIZE for a frame,
simply reallocate it; we're allowed to sleep in this path.

This finally allows us to accept 2160p streams (assuming your
GPU is fast enough, of course), at the cost of having to make
an OpenGL context for each decoder thread (because allocating
frames in GPU memory requires making a PBO and stuff).
Unfortunately, we still cannot decode 2160p DeckLink frames
because there's no OpenGL context there, but it's a good start.