Use browser font rendering instead of FreeType

This moves to using Emscripten's SDL implementation to render TTF fonts. This resolves a number of font rendering bugs (though probably adds some different ones), simplifies the font-rendering code substantially and makes font rendering lightning fast -- previously UIs would have visible pauses while waiting for FreeType. Diff also removes FreeType headers and .a for Emscripten.

I have also included code to use the same technique for texture loading. This works, however it breaks something else meaning that the EffectsTest no longer works correctly. I have #ifdefed it out for the time being, but this will yield similar performance improvements in startup time once it works.