vaapipostproc: use sink resolution to calculate src colorimetry

vaapipostproc: use sink resolution to calculate src colorimetry

The default output colorimetry is persuaded by the output
resolution, which is too naive when doing VPP cropping
and/or scaling.  For example, scaling 4K(sink)->1080P(src)
resolution (i.e. both YUV) results in bt2020(sink)->bt709(src)
colorimetry selection and some drivers don't support that
mode in vpp.

Thus, if output (i.e. downstream) does not specify a
colorimetry then we use the input resolution instead of the
output resolution to create the default colorimetry.  Also,
note that we still use the output format since it may be a
different color space than the input.  As in the example
above, this will result in bt2020(sink)->bt2020(src)
colorimetry selection and all drivers (afaik) should support
that in vpp.