vapostproc: Color fixation will choose othercaps' structure.

vapostproc: Color fixation will choose othercaps' structure.

gst_va_fixate_format() will iterate all othercaps' structures to find
the one with less information lost at color conversion. If a structure
with same color format is found, the iteration stops. It's like a
smart truncation. Then, this function also will choose the caps
feature.

Later this structure is used fixate its size and no further truncation
is needed.

Don't intersect at fixate, since it kills possible resizing.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1261>