videocontext: support wl-display in "gst.vaapi.app.Display"

Through "gst.vaapi.app.Display" context, users can set their own
VADisplay and native display of their backend.

So far we support only X11 display, from now we also support Wayland
display.

Attributes:
- wl-display : pointer of struct wl_display .

https://bugzilla.gnome.org/show_bug.cgi?id=705821

gst/vaapi/gstvaapivideocontext.c

@@ -31,6 +31,9 @@
 #if USE_X11
 #include <gst/vaapi/gstvaapidisplay_x11.h>
 #endif
+#if USE_WAYLAND
+#include <gst/vaapi/gstvaapidisplay_wayland.h>
+#endif
 
 GST_DEBUG_CATEGORY_STATIC (GST_CAT_CONTEXT);
 
@@ -103,6 +106,17 @@ gst_vaapi_video_context_get_display (GstContext * context, gboolean app_context,
             gst_vaapi_display_x11_new_with_va_display (va_display, x11_display);
       }
 #endif
+#if USE_WAYLAND
+      if (!display) {
+        struct wl_display *wl_display = NULL;
+        if (gst_structure_get (structure, "wl-display", G_TYPE_POINTER,
+                &wl_display, NULL)) {
+          display =
+              gst_vaapi_display_wayland_new_with_va_display (va_display,
+              wl_display);
+        }
+      }
+#endif
 
       _init_context_debug ();