gl4es feature detection does not work reliably with EGL.
If a context already exists, gl4es can instead reliably detect
the underlying GLES features from the context itself.
This requires gl4es to be configured with:
-DNOEGL=ON -DNO_LOADER=ON -DNO_INIT_CONSTRUCTOR=ON
This also requires gl4es to have this fix: https://github.com/ptitSeb/gl4es/pull/271