Revive the dummy interface

Updates the dummy interface to match current prototypes, and allows you to compile it via unsetting SDL explicitly via arguments to make (make SDL= or make SDL=0 works)
Note: Only compiles if you also pass NOHW=1. Should I make that automatically set in the Makefile? Probably.

I originally based this on master, but considering the code changes, I've decided to target next. That's why the branch is based on master. (It shouldn't break anything anyway.)