Multithreading 7/N: Add support to calling SDL 1.x from pthreads

[juj]

Adds support for calling SDL API from pthreads by synchronously proxying to main thread. This makes pthreads "see" the JavaScript state of SDL held in the main browser thread, so that multiple threads all act like they have one coherent state of SDL data structures. That allows one thread to call SDL_Init, and it will be in effect for all other threads as well.

[juj]

This depends on #5531 to land first.

[kripken]

How about using a loop at the end of the file to add all these annotations, instead of manually? We do that for other repetitive things.

[juj]

Setting in a loop would remove the foo__proxy: [...] part, but not the foo__sig part, and the information would need to be stored somehow still, via a sync_sdl_funcs = ['func1', 'func2' ...']; async_sdl_funcs = ['func3', 'func4'];. Then to understand the proxying model one would need to be jumping between two places in code, and to double check that each function indeed has an attribute specified and none of them missed it (not having an attribute is not an error, but calls in the current thread). I would prefer having these annotations where each function is, that seems like the cleanest way to read the code, even if a bit verbose?

[kripken]

Fair enough, lgtm.