Bug: Flickering in Sprite preview on Raspbian/Linux.

[indigodarkwolf]

For some reason, the Sprite preview flickers between the "correct" sprite data and seemingly garbage data.

To reproduce:

1. Start the emulator.
2. MOUSE 1 in the console.
3. Open the Sprite Settings overlay. (Windows -> VERA Debugging -> Sprite Settings)
4. Observe flickering. Some mouse movement might be required.