Fix sprite collisions; bus timings; sprite budget

[jburks]

This PR contains three minor changes:

1. A fix to the sprite collision logic for issue Is the negation of the sprite collision mask correct? #31
2. A bus timing improvement for more reliable functionality at 10MHz (still not working at 12MHz)
3. An adjustment to the layer memory access strobes to reduce VRAM arbitration when one layer is disabled (increases the available per-scanline cycles for rendering sprites).

All changes have undergone a few weeks of testing on both my mini-ITX hardware and the official X16 proto3 hardware, across multiple software programs. No issues were noted during this time.