The EGL test failures where provoked by two things:
 * GLES 2.0 deprecates `gl::TEXTURE_2D` as a valid parameter to
   `gl::enable`, which was provoking an unexpected INVALID_ENUM error.
 * GLES 2.0 doesn't have `glGetTexImage`, so to test that writing to a
   texture actually works we have to bind it to another FBO. We do it on
   the sharing test.

Also, added a no-op feature to easily run egl tests on linux.