Distinct, cleaned-up and fixed (hopefully) gaming PMOD verilog

[psychogenic]

Couldn't locate the VGA PMOD that I'm pretty certain I have, so I did this semi-blind, just using a scope, but the output is sensible and reacts to button presses as expected.

I have:

• made various fixes;
• isolated the gaming_pmod verilog from the sample project, so that it's reusable;
• changed things to support 2 controllers, have an is_present field to indicate detection, and leverage the 2-bit colours in the sample to actually show the state of both controllers with the bars;
• commented the module code to assist anyone actually using it

Also of note, something that cause quite a bit of putzing around, was we agreed on pins 4,5,6 but my snespmod docs are using PMOD indexing, and refer to ui_in[5:3], but this driver used ui_in[6:4]. I've left that, but need confirmation (and to edit) the CH32V lib accordingly.

[psychogenic]

I've updated the snespmod library docs, assuming we were talking TT ui_in numbering. Lemme know if this isn't what we want.