Retina window please {do not merge yet}

[dimitre]

closes #8329

[artificiel]

how does that interact with applications that rely on high-resolution rendering? (i.e. what is currently happening with the HIGH_RESOLUTION_CAPABLE = YES flag).

[dimitre]

I think there should be a boolean in ofWindowSettings.
like bool highResolutionCapable = false;
that can be changed in ofGLWindowSettings.

This PR isnt' working now anyway, there is something off on window size on master.
I've been using this other code, that is simplified and doesn't rely on pixelScreenCoordScale
https://github.com/dimitre/openFrameworks/blob/ofworks/libs/openFrameworks/app/ofAppGLFWWindow.cpp