Uniform resolution across backends

[piever]

The current situation with backends resolutions is the following.

• CairoMakie interprets (800, 600) as "units", so it means 600 x 450 pt or 800 x 600 px when embedded in a web site
• WGLMakie and GLMakie interpret (800, 600) as device pixels, which means they end up with tiny fonts on, e.g., retina displays

Possible solution is to use the resolution in CairoMakie (expressed in units) as the default, and adjust WGLMakie and GLMakie.

• For WGLMakie, it is actually straightforward, as it is easy to give the size in CSS pixels (we have now learned exactly all the bits where code needs to change)
• For GLMakie, it is slightly trickier, but I am curious whether we can keep the meaning of (800, 600) as units, and, depending on the pixel size, find a w x h such as (w, h) times the pixel size on that screen has the same physical dimension as 600 x 450 pt

This way, we would have by default consistent looks across backends. Especially a consistent font size, which now is problematic.

[piever]

Copying a comment from the slack chat, this could be further controllable by a px_per_unit = 1 in WGLMakie (as an attribute to Figure), and a device_px_per_unit in GLMakie.

[ffreyer]

With #2544 / #3113 (0.20) all backends now use generic units that are scaled with px_per_unit