st/xorg: render throttling in block handler

Similar to the classic swapbuffer throttling in GL drivers, put an
upper bound on the number of outstanding chunks of rendering the state
tracker can generate -- where calling the block handler denotes a
chunk.

Currently that number is set at around 4 "chunks", but could be
tweaked up or down.  If a better measure for the amount of outstanding
rendering is found, that would be fine too.

As it stands, this improves interactivity by preventing the X server
from queueing up arbitary amounts of rendering.

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