st/mesa: Enable Alpha writes when writing RGB faked as RGBA

st/mesa: Enable Alpha writes when writing RGB faked as RGBA

Some GPUs are able to render more efficiently when all channels of a
color attachment are written, since whole pixels are being overwritten,
rather than hitting a read-modify-write cycle where newly written data
has to be combined with existing unmodified image data.

When faking GL_RGB as RGBA (in case RGB/RGBX isn't color renderable),
we introduce an extra channel that doesn't exist from the application
point of view.  With such a format, a color mask of 0x7 (RGB) would mean
to write all channels.  But because we've added an alpha channel behind
their back, this becomes a partial write.  We are free to write whatever
garbage we want to the alpha channel, however.  So we can enable alpha
writes, making this a more efficient full pixel write again.

This is done unconditionally as it's expected to address a problem
common to many drivers and isn't expected to be harmful, even on GPUs
where it may not help much.

Improves WebGL Aquarium performance on Alderlake GT1 by around 2.4x, in
the Chromium, using Wayland (the --enable-features=UseOzonePlatform and
--ozone-platform=wayland flags).

v2: Don't require PIPE_CAP_RGB_OVERRIDE_DST_ALPHA_BLEND (Marek)
v3: Fix independent blending enables (Emma) - now set when needed,
    skipped when not needed, and PIPE_CAP_INDEP_BLEND_ENABLE is no
    longer a requirement.  We just optimize where we can.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7864
Reviewed-by: Matt Turner <mattst88@gmail.com> [v1]
Reviewed-by: Marek Olšák <marek.olsak@amd.com> [v2]
Reviewed-by: Emma Anholt <emma@anholt.net> [v3]
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20290>