icon-view: add back gtk_style_context_set_background()

icon-view: add back gtk_style_context_set_background()

Commit da09447914c0919362533182261a2c862ac8de81 removed the call to
gtk_style_context_set_background() in favour of always rendering it with
gtk_render_background() during the draw vfunc.
This has the side effect of making the backing window always
transparent, which blocks GTK from applying some optimizations during
the paint cycle. The result is that, especially in clutter-gtk
applications, scrolling performance gets really bad.

This commit partially reverts da09447914c0919362533182261a2c862ac8de81
and changes the code so that both gtk_style_context_set_background() and
gtk_render_background() are called.