Doomsday Engine

libgui: Drawing buffers with instancing, dynamic attrib locations

Added support for the ARB_draw_instanced and ARB_instanced_arrays
extensions to allow efficiently drawing multiple copies of a buffer at
once.

Vertex attribute locations are now dynamically determined by the linker.
This allows using a larger set of semantics, as only the attributes in
use need to have a reserved location. (There is a relative small number
of attribute locations available, plus matrix attributes need 4
locations each.)