Fix a bug where a drop-down list box is not correctly repositioned when clipped by the GUI viewport.
A slight change to the shadow style of the listbox of a drop-down box.
Increase the default resolution of directional shadow maps to 1024.
Clean up some redundant rendering calls from the GUi renderer.
Introduce the notion of utility functions to the renderer.

Currently, the only function here is to draw a fullscreen triangle with a given texture.
Update the demos.