Fixed|DMU: Added missing element/property name for DMU_SURFACE
libdeng2|BitField: Added assignment operator, method to get all element ids
libgui|GLState: Basic implementation of stacked GL state manipulation

GLStates can either be created on demand with GLState::push(), or
one can keep a GLState instance around for repeating use.

(Note that Direct3D uses an object-based state system compatible with
GLState.)