[2.6] YToUV filter should compute output colorspace from input clip dimensions when Y clip is provided

[avxsynth-testing]

In Avisynth 2.6, the YToUV function can determine the output colorspace from the input clip dimensions.

Consider:

y = BlankClip(width=640, height=480, pixel_type="YV12")
u = BlankClip(width=y.width, height=y.height/2, pixel_type="YV12")
v = BlankClip(width=y.width, height=y.height/2, pixel_type="YV12")
YToUV(u, v, y)

This returns a YV16 clip on Avisynth 2.6. On Avisynth 2.58 and AvxSynth, an exception is generated because the clip dimensions are not compatible with the YV12 format.

This depends on issue #79.

Link: issue #77.