glScissor state can now be saved on a stack

This gets usefull when ClipEntities are nested
Take a union of new and old clip settings when applying glScissor
Merge pull request #219 from nazgee/for-nicolas-scissor-fix

ClipEntities can now be nested.
Minor changes in GLScissorStack.
Minor change.
Added methods to easily convert from/to parent Entity coordinates.