compositor: q&d solution for surface drift

compositor: q&d solution for surface drift

When a transformed (rotated) surface is continuously resized from its
top-left corner, its location will drift. This is due to accumulating
rounding errors in transforming an offset from surface-local to global
coordinates in surface_attach().

Diminish the drift down to unobservable level by changing the
weston_surface global position from integer to float.

The offset transformation is now done without rounding. To preserve the
precision, wl_shell::configure() interface must use floats, and so does
weston_surface_configure(), too.

The con of this patch is that it adds inconsistency to the surface
position coordinates: sometimes they are floats, sometimes integers.