Replace Qt3D with scene graph API for map rendering.

QtLocation now links against QtQuick and the QGeoMap has a
updateSceneGraph(QSGNode *, QQuickWindow *) function to populate the
scene graph with map content in what ever style the implementation
decides. The only implementation right now is the QGeoTileMapData
(same as with the Qt3D code).

Camera positioning, zooming, panning, etc has been preserved in the
interest of minimzing the change and keeping things "as they were".

All rendering logic is moved into the updateSceneGraph function, where
we find the delta between what is shown in the scene graph and what
the scene things should be visible. Nodes are added removed
accordingly. This logic is isolated from the rest of the code as it
primarily lives in the scene graph render thread and may be released
at an arbitrary time. Doing this every frame might seem daunting, but
for the most, it is a couple of checks on a few tens of nodes,
compared to rendering millions of pixels afterwards, so this is not
going to be a bottleneck.

Texture uploads are now done in the scene graph's rendering phase, so
the map item's sync is fast. This means the map element doesn't block
both GUI and render threads for extended time periods. In contrast,
the Qt3D code path uploaded tiles and rendered them to an FBO in the
sync phase, resulting in very poor parallelization.

We cache the textures designated as visible and nothing
else. Primarily in the interest of simplicity, but also because
uploading 256x256 textures is a no-brainer on modern GPU's, including
lower-end mobile and embedded. The QGeoTileTexture sits on a QImage
instead of an actual texture to keep a strict separation between
render thread and GUI thread clean. Texture cleanup is done by the
scene graph structure.

The Qt3D codepath rendered to an FBO, so it was implicitly clipped to
its bounds. To preserve this behavior, the scene graph implementation
does the same. Not doing so looks a bit odd, at least when zooming all
the way out so the world is significantly smaller than the viewport.
The scene graph code also adds a rectangle of solid 0.9 gray to match
the default color of the old FBO code.

Change-Id: Ia736cea6e3616721c2de6beb5d72e570adcb21e6
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>