gsk/gl: use critical instead of assert in dispose

gsk/gl: use critical instead of assert in dispose

Instead of asserting only in debug builds (which are generally not
shipped in distributions) we should deliver a critical log-level message
so that these can be found sooner when not developing with jhbuild,
Flatpak, etc.

Also assert that we've setup the state correctly when realizing the
GskGLRenderer object.

Fixes #4625