fix depth-test for chest/block rendering in gui-slot elements.  partial
fix for render order of tooltips in crafting guis. (need to fix normal
item-stack rendering now)
more gui rendering updates.  fixes item-stack tooltip render-order
problems for _most_ guis (fix was in base class, so should propagate to
any derived guis).
(the depth-test issue was resolved in an earlier commit)

Closes #32