Adds support for SDL Window Events with event ids:
- `SDL_WINDOWEVENT_FOCUS_GAINED`
- `SDL_WINDOWEVENT_FOCUS_LOST`
- `SDL_WINDOWEVENT_SHOWN`
- `SDL_WINDOWEVENT_HIDDEN`

Includes a fix for FFOS phones that prevent the `blur` event when
audio is playing (Bugzilla 910340)

Conflicts:
	src/library_sdl.js
v1.18.2
do not move around loop variable incrementations when the helper is used outside the loop
add line bisector tool
fix emitsBoolean handling of conditional, and add handling of num
add conditional to hasSideEffects
add testcases for last few commits
Merge pull request #2362 from juj/sdl_windowevent_focus

Sdl windowevent focus
Implement Web Audio API playback backend to SDL Mix_* interface. This fixes https://github.com/kripken/emscripten/commit/5b41371bb5897a49e1fdcad18d053f775105c846 when Web Audio API is available, and works around the Firefox bug https://bugzilla.mozilla.org/show_bug.cgi?id=654787.