ARS-emu version 0.7-alpha

This release mainly fixes some bugs in the PPU emulation. Full changes:

• BGFG actually works correctly now.
• CRAM index #0 is no longer treated as transparent in sprites.
• Added a slightly spiffy, super slow VRAM visualizer to aid debugging.
• Improved sound quality a little bit by restoring the missing lowpass filter.
• Closed a hole that would allow nonstandard configuration code to run if the programmer was crafty enough.