GLX: attempt to fix glean makeCurrent test cases.

GLX: attempt to fix glean makeCurrent test cases.

Two parts to this:

One we don't keep pointers to possibly freed memory anymore once we unbind the
drawables from the context. Brian I need to figure out what the comment
you made there, can we get a glean/piglit test so we can fix it properly?

If the new gc is the same as the oldGC, we call the unbind even though
we just bound it in that function. doh.

(cherry picked from master, commit 77506dac8e81e9548a7e9680ce367175fe5747af)