Rewrite extension checking.

Rewrite extension checking.

Two big changes:

 1. If you're missing some functionality, Movit will now tell you
    on stderr what you're missing. (We might suppress this later
    if it turns out that people want to init_movit() but are actually
    fine with it failing.)
 2. Use a table instead of repeated if-then logic, since this started
    to become a bit messy after we added OpenGL-version-equivalence
    checks.