Merge lp://staging/~ubuntu-sdk-team/ubuntu-ui-toolkit/dpr into lp://staging/ubuntu-ui-toolkit

[RFC] Compensate for Qt's device pixel ratio multiplier

The UITK has flexible UI scaling support through the use of Grid Units, where one can set a grid unit to be an integer number of pixels, and the whole UI adopts to suit this. GRID_UNIT_PX=10 is the way to set this.

Qt however has its own scaling solution: QScreen::devicePixelRatio->qreal which is a multiplier applied internally to all sizing/positioning calculations. Only integer values are well supported, so 1,2,3... are the options, resulting in a x2, x3 scaling of the entire UI. For linux and X11, devicePixelRatio is specified with QT_DEVICE_PIXEL_RATIO=2. In this case, if a QML item is set to have height 100, it will be drawn 200 pixels high (but QML has no idea of this, it is still 100 in height)

This latter solution works for all Qt apps, whereas Grid units only apply to UITK-based apps. For a HighDPI desktop, we want to use both solutions.

However these two scaling solutions are cumulative. Should one set
QT_DEVICE_PIXEL_RATIO=2 GRID_UNIT_PX=10
then a box of height units.gu(1) will be drawn 20 physical pixels high.

The intention of this MR is to guarantee that GRID_UNIT_PX corresponds to physical pixels, no matter what QScreen::devicePixelRatio returns. To do this, it divides units.{gu,dp} by devicePixelRatio, so that when Qt multiplies it by devicePixelRatio again on draw, the desired physical pixel size is obtained.

To test, open the gallery with these 2 different envs:
QT_DEVICE_PIXEL_RATIO=1 GRID_UNIT_PX=10
QT_DEVICE_PIXEL_RATIO=2 GRID_UNIT_PX=10

Some visual inconsistencies are exact font rendering sizes and some rounding issues causing off-by-one positioning errors.