libyuv: add scaling support to YUV input before converting to RGB

[totaam]

See also 65bb2fd which allows cairo (the "non-opengl" rendering backend) to scale when painting.
This was needed for painting downscaled video.
But libyuv is probably faster than cairo. Worth checking too.

[totaam]

Needed the fix above to get the csc step to provide unscaled images to paint_image_wrapper method.

Tested briefly with glxspheres64 and --opengl=no on a 4k 60Hz monitor, looking at the fps values reported by the server:

• with cairo doing the scaling:

client.window.5.damage.fps=8
client.window.5.encoder.fps=8
client.window.5.video_subregion.fps=8

• with libyuv doing it:

client.window.6.damage.fps=15
client.window.6.encoder.fps=17
client.window.6.video_subregion.fps=14

It fluctuates slightly more, but it is also twice as fast.
Case closed.