vo_opengl: initial smoothmotion support

This uses a very simple algorithm that blends the resulting frames. The code
will need changes since the blending must be done in linear light to improve
image quality and color correctness.
vo_opengl: add a smoothmotion threshold

This allows to skip blending if the if there's a frame close enough to the
next vsync. A value of 0.2 will skip blending for frames that are within 20%
time distance of the vsync.