Polyfill glMapBufferRange and glUnmapBuffer from GLES 3.
Make sure to free the mapping slot.
Update AUTHORS.
Address review comments and add glFlushMappedBufferRange.
Move glMapBufferRange behind FULL_ES3.
Remove typo.
Merge branch 'polyfill-glMapBufferRange' of https://github.com/dvander/emscripten into incoming

Conflicts:
	AUTHORS