Fix data race when rendering an LCD display

[juhdanad]

###Summary

The example code for the LCD display uses two threads: one for simulating the AVR chip, and one for OpenGL. The state of the LCD display is stored as some flags in a uint16_t variable. When the OpenGL thread modifies a flag, it may accidentally undo a simultaneous write operation in the AVR thread, causing malfunction of the display.

The full description is available at buserror#507.

[juhdanad]

Sorry for force-pushing, I had to fix a minor mistake in the first commit, and I had to modify the first commit itself as it is required for testing the PR. I hope I was fast enough to not cause any trouble.

[akosthekiss]

Thanks for the patch. I have cherry-picked the important part of the "Add mutex to LCD display to prevent race condition" patch into the branch. Better late than never.