Added Scissor operations.

- Added scissor() and SCISSOR_TEST in various GL implementations.
- All UI elements now avoid to modify their Z position to introduce Z
levels later.
- Added a scissor test in UIList so that the text and items remains in
the list bounds.
- Added A ScissorStack class that handles nested scissor operations and
avoids GL gets.
- Added scissors operations in the TextLayer.
- Added a choice for UILayoutTwoPanes to layout in the original order
or filter order.
- Added a Globals class at root that handles various utility values
needed globally by programs.