Fix #875 - ES version should be strictly validated

When initializing the context in GLContextImpl.setGLFuncAvailability
ES devices must be validated by strictly matching the major version,
otherwise on ES3 devices we were mixing ES1 implementation with ES3
contexts, ultimately crashing in a safeguard.

Signed-off-by: Brice Figureau <brice@daysofwonder.com>
Merge pull request #72 from masterzen/tickets/875

Fix #875 - ES version should be strictly validated