Keep screen geometries from overlapping

The simple mapping of dividing each position by the
devicePixelRatio does not work when screens have different
DPR. If the low-DPR screen above or to the left of the high-DPR
screen, the geometries will overlap in the Qt coordinate system.

This change introduces a new mapping where the origin of each screen
does not move. This mapping is not perfect: it will have gaps between
contiguous screens. However, it will keep non-overlapping screens
non-overlapping in the Qt coordinate system.

Since there is no longer a simple linear coordinate transform, we
have to add screen-dependent mapping functions, and distinguish between
local and non-local coordinates. A side benefit is that the code is
now easier to read, since we remove most manual coordinate transformation.

We also have to cache the screen of each window: since we send resize events
before screen change events, we cannot rely on QPlatformWindow::screen()
(which is set from the screen change event).

Task-number: QTBUG-45076
Change-Id: Ie95a0b71ae274e02903caa102a98af2050a44129
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>