Blender Git Loki

Mac Retina fix:

Mouse coordinates were not mapped correctly for code that allows to use
multiple windows efficiently (mouse over not-active windows).

Apple's high-density display mode works a bit strange, requiring some hacks :/

- Desktop coordinate system (mouse pos and for windows) is as usual
(set by display resolution settings)
- However, the available pixels in a window is always on 'retina' level.
(full screen - 2880 wide, but window can be 1440 or 1920 wide)

In order to get this to work for opengl and Blender, we use internally the
coordinates on pixel level. That means that window positions and sizes have
to mappend in our code.

Once all issues for retinas have been tackled, I'll check on clean API for
it, so you can also use it in future for other high density screens.