Reduce size by removing SDL2 from build

[Valtteri Heikkilä]

Only audio is using SDL2 right now. Rendering and inputs are handled through emscripten APIs. Web Audio doesn't have such emscripten API so it would need a custom one using EM_ASM. Looking at the SDL2 port audio implementation, this would be quite a simple task.

Pros:

• Reduces size by probably 200K or more (non-gzipped).
• Removes SDL2 port dependency.

Cons:

• The new Web Audio code would require maintenance as SDL2 is no longer there.

[Valtteri Heikkilä]

Fixed in 0.3.0-alpha.