glx: Fix drawable lookup bugs in glXUseXFont

glx: Fix drawable lookup bugs in glXUseXFont

We were using the current drawable of the context to name the
appropriate screen for creating the bitmaps. But one, the current
drawable can be None, and two, it can be a GLXDrawable. Passing either
one as the second argument to XCreatePixmap will throw BadDrawable. Use
the root window of the context's screen instead.

Gitlab: https://gitlab.freedesktop.org/mesa/mesa/issues/89
LOLed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>