Do not use GDK Wayland module

pqiv has some rendering issues with this module, the same it had 8 years
ago when I first experimented with it. Since the x11 backend still works
just fine with Wayland, I see no need to fix this right now.

The issues are quite visible when running with GDK_BACKEND=wayland. See
also the TODO at the end of the draw callback.

README.markdown

@@ -185,6 +185,7 @@ Changelog
 pqiv 2.13 (dev)
  * Fix `toggle_fullscreen(1/2)` behavior when already fullscreen
  * Add `--font` to adjust info box font, use Pango for rendering (See #221)
+ * Prefer x11 over wayland GDK backend to avoid rendering issues
 
 pqiv 2.12
  * Fix external image filters (Fixes #182)

pqiv.c

@@ -8178,6 +8178,12 @@ int main(int argc, char *argv[]) {
 		#endif
 	#endif
 
+	#if GTK_CHECK_VERSION(3, 10, 0)
+		// Always prefer x11 over Wayland. Wayland rendering has several issues, in particular
+		// windows have an undesired black margin around them.
+		gdk_set_allowed_backends("x11,*");
+	#endif
+
 	#if defined(GDK_WINDOWING_X11)
 		XInitThreads();
 	#endif