This was an ill-advised extension. While we advertised SWAP_COPY support,
we might implement it with a back-copy from the front buffer. And we
never advertised EXCHANGE because we couldn't guarantee it. So, if you
actually used this extension to try to reduce app redraws of the back
buffer, you might actually increase the bandwidth you used. Whoops.
Instead, GLX_EXT_buffer_age and the similar EGL extension give you
feedback on what's left in your back buffer, letting you do minimum
redraws.
This reduces our GLX visual+fbconfig count from 1410 to 940 on an llvmpipe
X server. Reducing visual counts will improve test runtime for
visual-iterating tests like piglit's glx-visuals-*.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25650>
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