compositor/blend: fix blending of subsampled components

compositor/blend: fix blending of subsampled components

The correct way to determine the byte offset at a certain yoffset
in a subsampled component is to shift the yoffset by the component's
hsub

This fixes out-of-bounds memory accesses and visible artefacts,
example pipeline with the samples from #802:

gst-launch-1.0 compositor name=vmixer sink_1::xpos=1910 sink_1::ypos=1080 ! \
    videoconvert ! videorate ! xvimagesink \
  filesrc location=VID_20200723_203606.mp4 ! decodebin name=demux1 ! \
    queue ! videoflip method=vertical-flip ! vmixer. \
  filesrc location=bridgeoverstubbledwater.mp4 ! decodebin name=demux2 ! \
    queue ! vmixer.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/988>