Emscripten sdl set event handler

Adds a new extension function 'emscripten_SDL_SetEventHandler' to SDL to allow routing input handling to occur inside a browser event handler. This enables Emscripten SDL code to perform functions that require being invoked from within an event handler (fullscreen request, pointer lock, payments, etc.) without the need to defer the processing to the next time an event handler is run.
Oh, didn't work out like discussed on IRC - that would require pulling in SDL header to emscripten.h.