surface-actor-x11: Make sure to set a size when unredirected

When we're unredirected, we don't have a pixmap, and thus our allocation
becomes 0x0. So when events come in, they pass right through our actor,
going to the one underneath in the stack.

Fix this by having a fallback size on the shaped texture actor when
we're unredirected, causing it to always have a valid allocation.

This fixes clicking on stuff in sloppy / mouse mode focus.