Memory Leak: Takes up all memory in system when displaying large images at high rate.

[nstelter-slac]

PyDM could take all memory in the system quite rapidly when displaying large images at high rate.

Note that this memory leak didn't appear all the time and it was not obvious what was triggering the behavior.

earlyoom (https://github.com/rfjakob/earlyoom) had to be installed on all consoles to prevent pydm from crashing the system.