These formats are useful because they are often easier to produce
on CPU than half-float formats, and abgr16161616 has both >= 10bpc
color channels and adequate alpha, unlike abgr2101010.
The 16-bpc textures created from buffers with these formats require
the GL_EXT_texture_norm16 extension.
As WL_SHM_FORMAT_ABGR16161616 was introduced in libwayland 1.20,
update Weston's build requirements and CI.
The formats also needed to be registered in the pixel format table,
and defined in a fallback path if recent libdrm is not available.
Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>
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