optimize item-stack/slot rendering for gui-element. FPS with warehouse
gui open now is close to on-par with the creative inventory gui being
open. Most of these optimizations should also effect other
Ancient-Warfare GUIs as they are all based on the same code.  This was
possible because of the recent time I've spent learning low-level OpenGL
stuff.

Anything with a scroll-pane will now only render those child-elements
which are at least partially visible (would previously render every
element in the scroll pane, even if not visible).  Also removed many
push/pop attrib statements -- which is a horribly inefficient way to
accomplish what I was trying to do -- replaced with more appropriate
context-specific calls.

I've likely missed a GUI or two that may need a change in rendering code
to handle the optimizations as I was previously relying on the push/pop
attrib far too much.

Closes #4