Analogous to the light() function in state.c, the correct ModelView
matrix has to be loaded when glLightfv is called.

This will allow some color keying cleanups. For Core Contexts we'll have
to emulate alpha testing in shaders anyway.

This is necessary if e.g. the nvts or ffp fragment processing pipeline
is selected for debugging purposes on a GPU that otherwise supports the
ARB program blitter.