This allows us not to have to hack our definitions in the upstream files,
making it easier to upgrade to newer versions in the future.
For the include paths to work, the headers are moved to a GL subfolder to
match their upstream location.
-=-=-=-=-=-=-=-=-=-
-Moved drawing code to a single function that takes linked list (should make it easier to optimize in the future).
-Implemented Z ordering of 2D nodes. Node2D and those that inherit have a visibility/Z property that affects drawing order (besides the tree order)
-Removed OpenGL ES 1.x support. Good riddance!
