st-texture-cache: Use StContentImage to create actor contents

st-texture-cache: Use StContentImage to create actor contents

This will allow to use resource-scale aware content that will use a preferred
size that takes in account the icon scaling and the resource scaling.

If no size is passed we should just divide the actual resource size by the
resource scale, as we assume that it's loaded with such scaling applied.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/5
https://bugzilla.gnome.org/show_bug.cgi?id=765011