There's no need to keep track of the window which last performed a
Present flip. This fixes crashes due to the assertion in
xwl_present_flips_stop failing. Fixes issue #10.
The damage generated by a flip only needs to be ignored once, then
xwl_window::present_flipped can be cleared. This may fix freezing in
the (hypothetical) scenario where Present flips are performed on a
window, followed by other drawing requests using the window as the
destination, but nothing triggering xwl_present_flips_stop. The damage
from the latter drawing requests would continue being ignored.
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