Commit 8d67186cb2 caches whether the device
transform is identity on context creation. However, the api is quite lax
and allows the user to modify the device transform *after* he has
started to use the surface in a context, as apparently WebKit does.
Since this is not the only instance where we may need to invalidate
caches if the user modifies state, introduce a simple mechanism for
hooking into notifications of property changes.
Fixes test/clip-device-offset.
