stage: Fix indentation in pick methods

https://bugzilla.gnome.org/show_bug.cgi?id=712563
stage: Remove the pick buffer caching

Since the journal is flushed on context switches, trying to use a cached
buffer means that we will use glReadPixels when picking, which isn't what
we want. Instead, always use a clipped draw, and remove the logic for
caching the pick buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=712563