gl/caopengllayer: use public GstGLContext instead of Cocoa-specific one

Allows keeping the GstGLCAOpenGLLayer public but not the winsys-specific
context/display/window.
diff --git a/ext/qt/gstqtglutility.cc b/ext/qt/gstqtglutility.cc
index ff8d875..4a88bb6 100644
--- a/ext/qt/gstqtglutility.cc
+++ b/ext/qt/gstqtglutility.cc
@@ -51,10 +51,6 @@
 #include <gst/gl/egl/gstglcontext_egl.h>
 #endif
 
-#if GST_GL_HAVE_WINDOW_COCOA && GST_GL_HAVE_PLATFORM_CGL && defined (HAVE_QT_MAC)
-#include <gst/gl/cocoa/gstgldisplay_cocoa.h>
-#endif
-
 #define GST_CAT_DEFAULT qt_gl_utils_debug
 GST_DEBUG_CATEGORY_STATIC (GST_CAT_DEFAULT);
 
@@ -126,7 +122,7 @@
 
 #if GST_GL_HAVE_WINDOW_COCOA && GST_GL_HAVE_PLATFORM_CGL && defined (HAVE_QT_MAC)
   if (QString::fromUtf8 ("cocoa") == app->platformName())
-    display = (GstGLDisplay *) gst_gl_display_cocoa_new ();
+    display = (GstGLDisplay *) gst_gl_display_new ();
 #endif
 #if GST_GL_HAVE_WINDOW_EAGL && GST_GL_HAVE_PLATFORM_EAGL && defined (HAVE_QT_IOS)
   if (QString::fromUtf8 ("ios") == app->platformName())