vo_opengl: greatly increase smoothmotion performance

Instead of rendering and upscaling each video frame on every vsync, this
version of the algorithm only draws them once and caches the result,
so the only operation that has to run on every vsync is a cheap linear
interpolation, plus CMS/dithering.

On my machine, this is a huge speedup for 24 Hz content (on a 60 Hz
monitor), up to 120% faster. (The speedup is not quite 250% because of
the overhead that the larger FBOs and CMS provides)

In terms of the implementation, this commit basically swaps
interpolation and upscaling - upscaling is moved to inter_program, and
interpolation is moved to the final_program.

Furthermore, the main bulk of the frame rendering logic (upscaling etc.)
was moved to a separete function, which is called from
gl_video_interpolate_frame only if it's actually necessarily, and
skipped otherwise.

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