opengl: handle downscaled luminance components

Commit d8aebc36 said:

The texture scaling factor represents how a plane is scaled compared to the first plane, so by definition, the first plane is not scaled (its ratio is 1:1).

However, with YUY2 pictures, half the horizontal resolution is currently lost by the interop. Therefore, until this is fixed, the sampler must scale accordingly.

This reverts commit d8aebc36.

Fixes #26682 (closed)

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To generate a YUY2 sample and play it (thanks to @chub):

gst-launch-1.0 videotestsrc num-buffers=100 ! video/x-raw,format=YUY2,width=320,height=200 ! filesink location=yuy2.raw
./vlc --demux rawvideo --rawvid-fps 25 --rawvid-width 320 --rawvid-height 200 --rawvid-chroma=YUY2 ./yuy2.raw