Some minor accuracy fixes pertaining to audio & rendering

Hello,

I've added some changes to fix a few of the minor emulation inaccuracies I've noticed. Here are the main ones:

The functionality of the BG clipping and Sprite clipping now work independently of each other, and the leftmost column now shows the BG color instead of a solid black (except for when TV clipping is enabled).
Interpolation has been removed from the triangle channel so that it retains its characteristic "stair-step" wave pattern.
Fixed an issue with the rightmost column of the screen not rendering correctly when TV clipping is disabled.
All APU channels are now center-panned by default.
NTSC palette modified to align more closely with the actual NTSC colors.

I've also added on a few small extra features:

In addition to the APU panning table, I've added an APU volume mixer table, plus a table of "channel enable" flags, intended to be enabled/disable by the user via a GUI (and function independently of the register-controlled isEnabled flags).
Added an "onFrameStart" callback option to the NES object, which is triggered when the top scanline is rendered as opposed to the bottom scanline like "onFrame".
The TV clipping PPU flag has now been split into 2 separate flags for horizontal and vertical clipping. Only vertical is enabled by default.

bfirsh / jsnes