OPENGL_GAL: Init currentTarget

The currentTarget member of OPENGL_GAL is not initialised, but the
currentManager member is.

This commit uses SetTarget to initialise both target and manager to a
consistent state at construction.

common/gal/opengl/opengl_gal.cpp

@@ -140,7 +140,7 @@ OPENGL_GAL::OPENGL_GAL( GAL_DISPLAY_OPTIONS& aDisplayOptions, wxWindow* aParent,
 
     gluTessProperty( tesselator, GLU_TESS_WINDING_RULE, GLU_TESS_WINDING_POSITIVE );
 
-    currentManager = nonCachedManager;
+    SetTarget( TARGET_NONCACHED );
 }