[regression] NV12 dmabuf renders all red

Originally found using the new vaapi-wayland video output from mpv: https://github.com/mpv-player/mpv/issues/10341

I bisected this issue to f36d77a1.

Reverting this commit or changing line 2573 to SHADER_VARIANT_EXTERNAL fixes this issue.

I’m not sure whether this is a bug in Weston or in Mesa/iris, which is the GL stack I’m using.

I’m also surprised Weston doesn’t try to put this buffer in a plane, as my hardware (gen9) supports everything needed here I think.