Use screen_block_draw to clear prefs area

[rhalkyard]

Based on @greg-king5's comment on PR #40, I reworked prefs_clear to just use screen_block_draw, which does all the scaling for us, and got rid of all the redundant scaling logic that I added (and the special-case ifdef for the Apple 2).

As a side effect, since screen_block_draw asserts flow control, it was necessary to add a dummy flow-control function, that the flow on/off function pointers can point at until the drivers are loaded.