gtk3: Stop using pixdata resources

Using 'to-pixdata' in gresources has been deprecated since gdk-pixbuf
version 2.32 [1]. Using it also requires having 'gdk-pixbuf-pixdata'
command available on the build environment, which may be part of
various different packages split from gdk-pixbuf depending on
distribution, and is therefore difficult dependency to document.

[1] https://developer-old.gnome.org/gio/unstable/GResource.html

Author: jnsh

common/gtk-3.0/meson.build

@@ -77,8 +77,8 @@ foreach variant : get_option('variants')
 
   foreach asset : gtk3_asset_names
     gresource_xml_array += [
-      '<file preprocess="to-pixdata">assets/' + asset + '.png</file>',
-      '<file preprocess="to-pixdata">assets/' + asset + '@2.png</file>'
+      '<file>assets/' + asset + '.png</file>',
+      '<file>assets/' + asset + '@2.png</file>'
     ]
   endforeach