The shadows are currently rendered by painting the actor we want to
apply shadow on, in an offscreen buffer. The problem is that when this
actor has an allocation padding (ie allocation that isn't at 0x0
relatively to its parent), this padding is added within the offscreen
buffer and as a result the shadow rendering is truncated because the
offscreen buffer size is the size of the allocation box, not the
allocation box + padding.

This patch reposition the actor at 0x0 with rendering it by changing
the initial transformation matrix when rendering the actor offscreen.

https://bugzilla.gnome.org/show_bug.cgi?id=698301