Add client which uses up finite compositor resources

Created by: mstoeckl

See the commit message for a longer description.

This is an adversarial client which makes it easier to test what happens when finite compositor resources run out.

An interesting combination is running 'resource-thief map' followed by 'resource-thief mem', which can make some memory allocations fail. In practice, the 'resource-thief fd' mode is probably most useful, since the scenario it represents can actually happen if you accidentally launch a GUI program in a loop.

It adds an optional dependency on libgbm.