Direct2D QPA: Cache QVectorPaths if drawn more than once

Hooking into the caching mechanism gets us a measurable performance boost
for paths that are drawn repeatedly, around 10% on my machine when drawing
aliased arcs.

Change-Id: I32f4ed7daa8a51b5c5a9c6d5414ab5d4ef759f70
Reviewed-by: Risto Avila <risto.avila@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Direct2D QPA: Get rid of QPainterPath conversion function

This function was used only in one place and duplicated a lot of logic
with the very similar QVectorPath conversion function. Just use
QVectorPath everywhere instead.

Change-Id: I3a4821f0452634c309ca0730047ea6ef7a7591ca
Reviewed-by: Risto Avila <risto.avila@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Direct2D QPA: Move QVectorPath conversion function into private class

The function already needs information from the engine for conversion,
moving it into the private class makes it possible to get this info
directly in the method and makes calling the method nicer.

Change-Id: I47fa9a4531d0d0605aa587fba90fbfdf036f0998
Reviewed-by: Risto Avila <risto.avila@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>