GL screensavers fail to load Mesa renderer add-on

The screensaver add-ons are properly linked against libGL. The libGL
code tries to load the swrast add-on. But this fails because the
BGLRenderer constructor (which is provided by libGL) is not found.

It seems that when loading an add-on, libraries linked to by other
add-ons (and not the app itself) are not searched to resolve symbols. To
avoid this issue, we now link ScreenSaver and screen_blanker to libGL,
so the GL renderer add-on can find it.

Fixes #10206
Update mesa package to 7.9.2-9

* After testing the previous fix, I found that Flurry would crash again
because of a stack overflow in Mesa. This new package fixes that other
issue, so Flurry runs again.
* GLife and Gravity are still crashing, however. I'll debug these next.
Gravity and GLife: fix crash on start.

Locking the GL context from a GLView subclass constructor can't work, as
it isn't ready yet. Move the initial setup to AttachedToWindow instead.

Fixes #8898, #10469.