backends: Use latest pointer position if no pointer constrain exists

The meta_seat_native_constrain_pointer() function receives the current pointer position, and the new pointer position as in/out parameters. We were however calculating the new coordinates based on the last pointer position if there was no pointer constrain in place.

Fortunately to us, this didn't use to happen often/ever, as a pointer constrain function is set on MetaBackend initialization. This behavior did also exist previously in MetaDeviceManagerNative.