Use a newer FlextGL that is more compatible with OS X.

Unfortunately, we seem to be useless on OS X for the moment because
GL_ARB_texture_storage is not supported. We might need to implement that in
terms of glTexImage2D.
Implement a fallback if GL_ARB_texture_storage is missing.

The fallback might not be perfect but it will allow us to work on OS X that
doesn't have this extension. Now we don't require any non-ubiquitous extension!