asahi: Impose limits on resource shadowing

asahi: Impose limits on resource shadowing

Apps can have pathological use cases where huge resources are shadowed
repeatedly. An app that alternately writes to a resource and then uses
it to draw can create an unbounded amount of shadow BOs.

To fix this, introduce both a maximum resource size for shadowing, and a
maximum cumulative size that resource may be shadowed before we start
flushing readers. The flush path then clears the counter, as does the
happy path where there are no readers left after flushing writers.

Fixes massive memory bloating in Firefox and probably others.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24635>