Add support for conditionally excluding all handwritten JS code for SDL.

[juj]

Currently we have SDL code in at least src/library_sdl.js and some also in src/library_browser.js. Alon added link flags -s NO_FILESYSTEM=1 and -s NO_BROWSER=1 in relation to #2545 , and in the same style, we should add a link flag -s NO_SDL=1 or similar, which would strip out all the handwritten JS code that implements Emscripten's SDL 1.3.

The motivation for this is with our SDL 2 implementation, which seems to have some bits still using these handwritten JS paths, so this flag would help explicitly confirm that the SDL 2 port would properly use only the SDL2 code. Also, this would secondarily benefit other codebases to allow them explicitly opt out of linking to SDL at all.

[kripken]

Also we should make -lSDL when linking turn it on, and default it to off.

[kripken]

Note that using SDL_mixer from our JS code is possible with SDL2. So this flag would only need to limit core SDL stuff, I suppose.

[stale]

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