Allow direct uploads of 16-bpc QImage formats

This MR is related to (but can be applied independently of) kwayland-server!350 (merged) .

In short, kwayland-server!350 (merged) allows Wayland clients to submit shared memory buffers with certain 16-bit-per-channel pixel formats. The commit from this MR ensures that kwin can directly upload such buffers as GL textures, instead of lossily converting them to ARBB32 format and then uploading them.

Regarding testing: since the new format codes were only added in the recent libwayland 1.20, no regular applications can use them yet, and only Weston has added support so far. To check that this MR works correctly, I used a test client for buffer formats that I wrote a few months ago.