Project quads to z=0 instead of compressing them between 0 and 1

Since all quads are already separated in non-intersecting geometries
and ordered according to their depth by Chromium, there is no need to
keep any z value once the perspective projection has been applied.

Avoid the useless computation and project the quads to z=0 the same
way that QMatrix4x4::toTransform() does when used to flatten
QQuickItem::transform().

Change-Id: I3ec5dc2da23cf431e2d8d0bb8b1ed48f2f2d2d8e
Reviewed-by: Andras Becsi <andras.becsi@digia.com>