Doomsday Engine

API|GL: Public GL API now uses GLState for scissor; added push/pop state

This commit fixes scissor-related issues when drawing stuff via the
public GL API (e.g., game automap).

The public GL API now internally uses GLState to set the scissor box.
This fixes a GL state update conflict between legacy code and libgui
based code. The public GL API now also has functions for pushing and
popping GLState.

The version number of the public GL API was incremented; old API is
no longer supported.