glcolorconvert: try harder to passthrough

glcolorconvert: try harder to passthrough

This makes a pipeline like:

... ! video/x-raw(memory:GLMemory),format=UYVY ! glcolorconvert !
video/x-raw(memory:GLMemory),format={UYVY, NV12} ! ...

passthrough instead of converting UYVY => NV12. The conversion would happen
before this change since the element (and basetransform) transform the src caps
to format={NV12, UYVY} (since NV12 comes first in the glcolorconvert:src
template) and then the default caps fixate func would fixate to NV12. Blah.

Also there's no need to intersect against the template caps in ::transform_caps
since basetransform does that right after calling the vfunc.