Need to implement the ability to (semi-)arbitrarily size sketches (both displayed output and re-scaling mouse coordinates) on HiDPI screens.

There is another issue opened for the Linux-specific parts of the problem: processing/processing4#407
As well as the Windows-specific parts of the issue: #4894

This issue is about how it should be handled by the renderers within processing.core.

In particular:

• it needs to be handled across the JavaFX, OpenGL, and default (Java2D) renderers
• we should figure out how pixelDensity() works in these situations
• this is an arbitrary scaling thing, which could also be a useful feature, but that can be considered separately