First pass at 0x00406C96

Hey,

This is my first time playing around with OpenRCT2 and my first time working with x86 ASM in a long time, so look over things carefully :). I've written a very basic first pass of this simple function - structs/globals/etc could possibly be added, I left comments about what I think is going on.

What does testing look like? I've played around with it manually and things seem to work fine, but that is never a great test :)