The "webgl/webgl2.h" include provides that function, but it's not
available in emscripten versions < 2.0.17 .
Since we need to support emscripten 1.39.9 (mono builds), this commit
adds a JS function in library_godot_display.js as a compatibility layer
for it, and implement glGetBufferSubData by funneling the call to that
function (so we don't have name collisions JS-side with recent emcc).
All those hacks are now moved to the platform directory instead of being
ifdefs inside the drivers implementations.
Fabio Alessandrelli