Fix menu skipping caused by "Overclock when busy"

[iczero]

When the "overclock when busy" option is enabled with TI-89 family calculators, using the arrow keys in menus will cause skipping. This appears to happen because turbo kicks in too soon, resulting in the arrow keys triggering the calculator's autorepeat. This seems to be fixed simply by waiting 2 frames before starting turbo. Menu scrolling usually seems to complete in a single frame.

Also, thanks for reviving the project.

[iczero]

Looks like the root cause is not this. Will look further

[iczero]

@eanema Do you have any objections to simply removing the screen update thread? As far as I can tell, there is no performance penalty from running screen refresh within the emulation thread. correction: yes there is. Merging the screen thread also seems to reduce flickering and other artifacts. I assume they were originally separate threads since phones 10 years ago were slow.

The root cause of the menu skipping issue is that isBusy is only updated from the screen thread, but the screen thread runs at a constant rate (SCREEN_LOOP_SLEEP) irrespective of whether the emulation thread is turboing. As such, if the emulation thread begins turboing, it won't stop until the screen thread next wakes up, even if the busy status was cleared.

iczero@bc2cf0c