create ShellRenderInterfaceOpenGL::SetViewport, instead of hardcoding

the height of the screen for scissoring. anything that would overflow
y with scrollbars would result in being cut off by a the difference between
768 and the viewports current resolution. even the debugger was broken.

change all examples to use this now as well.
don't render debug outlines on non-visible elements
Merge pull request #94 from sreich/master

opengl -- don't hardcode the height of the screen to 768 for scissoring