cogl-context: Use the function pointer for glActiveUnit
cogl-context: Use the function pointer for glActiveUnit

Under WGL, any functions that were defined after GL 1.1 are not
directly exported in the DLL so we need to reference them via the
function pointers. A new call to glActiveUnit was missed in
cogl-context.c

djdeath/cogl,collects/cogl,Distrotech/cogl,djdeath/cogl,gcampax/cogl,Distrotech/cogl,djdeath/cogl,Distrotech/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl-android,spatulasnout/cogl,Distrotech/cogl,collects/cogl,djdeath/cogl-android,djdeath/cogl-android,gcampax/cogl,spatulasnout/cogl