[Development] Landing support for high-dpi Qt 5

[Sorvig Morten]

Here are the changes:

https://codereview.qt-project.org/#change,40096
https://codereview.qt-project.org/#change,40215

And screenshots:

http://imgur.com/a/Zs6JJ
(You probably want to select the "view in full resolution" option for those)

This is a subset of the larger high-dpi patch presented earlier, with minimal added API and minimal behaviour changes.

The main reason for committing this now is to close the feature regression from Qt 4 to Qt 5. Qt 4 gets high-dpi support via the CoreGraphics paint engine. Qt 5 uses raster and we need to do the implementation work in Qt.

In addition the patches also implement support for high-dpi OpenGL and Scene Graph/Qt Quick 2. There are also patches to the fusion style. These changes are relatively small with high impact so I don't think it makes sense to hold them back.

As you can see from the commit messages I've changed terminology and adopted "device pixels", "device independent pixels", and "devicePixelRatio" from the web world. This builds on existing API naming and avoids vendor-specific overloaded terms like "Points".

There is no new mac-specific API added. This is intentional, and the implementation is also mainly done in cross-platform code. We're already using this patch as a basis for high-dpi support in the iOS port. (Although that platform is not a very big leap from Mac OS.)

Morten