Fixing retina screen display and allowing to save upscaled GLMakie images

[jkrumbiegel]

I think both of these issues are connected, and I'd like to get some help in figuring out the solution.

Basically, currently there is no implementation for retina screens. What happens is that when it's detected that a screen has a retina / hi-dpi factor, the window resolution is reduced by that factor. On Mac at least that means that a 800x600 window is requested as a 400x300 window. Because of retina framebuffers, the backing buffer is actually 800x600 again, but because the effective window size is 400x300, the window is small and text is tiny.

In order to fix this, I think we'd need a way to decouple framebuffer and window size. This means, one should be able to draw any window at an arbitrary framebuffer resolution. For retina windows, one would choose 2 as the correct factor, but for saving out high-resolution images instead of being limited to monitor resolution, one could choose 3, 4 or 5 (not displaying that, just rendering it).

However, I don't understand the pipeline well enough to implement it, in my preliminary tests I only managed to correct the window size on Mac, but things were still too small visually.