Improved Execution Times for Two Paint functions, and GetHexString.

By moving repeated calls to measure the same string with the same font, I was able to reduce the paint time of MemoryDisplay_Paint from 12ms per call, to 7ms per call. A similar magnitude of improvement was made by doing the same in DisassemblyDisplay_Paint.

Storing the brush colours outside the loop in MemoryDisplay_Paint did seem to have an effect on execution time, but it didn't in DisassemblyDisplay_Paint.

Additionally, the changes made to GetHexString more than doubled the speed of execution.