[QGIS-Developer] Incorrect hidpi setting in 3.0 for OSX

[Nyall Dawson]

Hey all (specifically OSX devs)

>From a few open bug reports we have against the OSX build of 3.0, I
gather that the OSX builds currently are not correctly handling hidpi
screens.

My understanding is that the builds are silently trying to scale the
UI by pixel doubling/scaling - it's resulting in bugs like
https://issues.qgis.org/issues/18043,
https://issues.qgis.org/issues/17773, and
https://issues.qgis.org/issues/15984.

Does any OSX expert know why this is happening? I think the automatic
pixel scaling should be switched off - while master isn't perfect with
hidpi screens, it's usable and the benefits of the hidpi
canvas/composer outweigh the remaining issues.

Plus, I'd rather we use a single approach to hidpi scaling then have
to worry about separate code paths for OSX.

Nyall