[scene] Introduce helpers for mapping between different coordinate spaces

We currently deal with three distinct coordinate spaces - the window
pixmap coordinate space, the window coordinate space, and the buffer
pixel coordinate space.

This change introduces a couple of helper methods to make it easier
to map points from the window pixmap space to the other two spaces.

The main motivation behind the new helpers is to break the direct
relationship between the surface-local coordinates and buffer pixel
coordinates for wayland surfaces.