Cleanup|Renderer: Bloom shader definition
OS X: Support HiDPI display resolutions

This commit makes Doomsday aware of HiDPI display resolutions on OS X.
DisplayMode.DPI_FACTOR can be checked to see what is the current
device pixel ratio (2 for HiDPI, otherwise 1).

Tweaked various widgets in libappfw so that hardcoded pixel sizes are
scaled to device pixels as appropriate.

Canvas was updated to translate mouse coordinates to device pixels.

The default UI style applies DisplayMode.DPI_FACTOR for rules, fonts,
and images.