Emscipten update

With the switch to SDL3 I was able to convert the engine to support SDL3's main callbacks. Once that was finished I simply had to setup an Emscripten build to support web deployment. Now Pyromancer can be played in the browser, plus any other game using Umbra that I feel like porting.

The changes to the engine needed to support this have swapped all of the old bugs for new ones. The old engine used the poorly designed libtcod event API which causes lag whenever the SDL event queue backed up. This engine, and a lot of programs by this developer, love to store events before operating on them later which means that when multiple events are triggered per-frame then many can be skipped before the update step checks which event it has. Fixing this requires more of an overhaul.

I tested the game and played it to completion. The game became a lot more stable after ensuring that bounds checks use signed integers.