Add dmabuf tests to exercise color format conversion shaders

The GL-renderer supports a good bunch of pixel formats, and it has a selection of shaders to manually convert YUV to RGB.

Write some screenshooting-based tests that prepare a wl_buffer with a given pixel format, commit it into a window, and take a screenshot to ensure it shows up correctly. Do this for such pixel formats that you get most, if not all, fragment shader pieces in GL-renderer exercised.

This likely requires creating zwp_linux_dmabuf buffers manually. See e.g. simple-dmabuf-egl.c.

#354 takes care of the usual RGB formats, those do not need to be tested here.

This depends on #310 (closed) for infrastructure.