Implement workaround for Mesa issue

This addresses the issue where systems running Mesa drivers would be an
order of magnitude slower than other terminals. With this patch,
performance is nearly as good as with proprietary drivers. There is one
caveat where, since the cause of slowness hasn't been removed, there is
less time available for rendering on Mesa systems.

I've benchmarked this on my system (i5-3570 @ 3.4GHz, GTX 680) using
`time find /usr` with the following results:

* ~2.0 seconds average with proprietary driver
* ~2.3 seconds average with Mesa driver

cc #125

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