Grid 64

[ergodicbreak]

Modifies grid support for 8x8 grids.

I've highlighted some questions.

Changes:

• code depends on variable grid_width instead of fixed number 16
• 64 grids have 56 sample slots available
• grid beat resolution is half beats rather than quarter beats (e.g. grid button [0][8] is beat four instead of two).

Not changed:

• Sample loading; I'm assuming folders with more than eight samples are OK to load onto rows of 64 grids
• Pset saving; I'm not sure if psets saved from a 64 grid session will load OK onto a 128 grid session, specifically because of beat resolution

I've commented the code (with TODO, DONE, and CHECK) and will clean everything up once the PR looks good.

Also I'm not that savvy with Github and pull requests and am a Lua beginner, so if I've messed anything up I am more than happy to correct it! Thanks.