The current ARGB2101010 has really "corase" control over the alpha.
Particularily, examples/layer-shell would look really strange with certain
parameters. For examples, when passing an alpha of 0.84, the box would not
appear transparent at all anymore.
Patched as suggested by @mstoeckl -- thank you!
The specified clients in this commit used to rely on wlr_egl and
some of its related functions in order to render surfaces.
This is no longer the case as of this commit.
