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

cogl/cogl-context.c

@@ -37,6 +37,10 @@
 
 #include <string.h>
 
+#ifdef HAVE_COGL_GL
+#define glActiveTexture _context->drv.pf_glActiveTexture
+#endif
+
 extern void
 _cogl_create_context_driver (CoglContext *context);