videoconvert: Compare y offset with height, not width, when testing for overlap
videoconvert: Compare y offset with height, not width, when testing for overlap

This could have prevented images showing that should have when the
source height is greater than its width.

When width exceeds height, as is common, it probably only caused a
miniscule amount of unnecessary work.  I haven't tested.

Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good