cogl-pipeline-opengl: Fix enabling previously disabled units

When enabling a unit that was disabled from a previous flush pipeline
it was forgetting to rebind the right texture unit so it wouldn't
work. This was causing the redhand to disappear when using the fixed
function backend in test-cogl-multitexture if anything else is added
to the scene.

cogl/cogl-pipeline-opengl.c

@@ -725,6 +725,7 @@ flush_layers_common_gl_state_cb (CoglPipelineLayer *layer, void *user_data)
       if (!G_UNLIKELY (cogl_debug_flags & COGL_DEBUG_DISABLE_TEXTURING) &&
           !unit->enabled)
         {
+          _cogl_set_active_texture_unit (unit_index);
           GE (glEnable (unit->current_gl_target));
           unit->enabled = TRUE;
         }