This implements and exposes the `View`/`Scene` brush radius option.
* `View`: Brush units are in pixels. Zooming in and out in the viewport will make the brush stay the same size relative to the view, but grow/shrink relative to the scene.
* `Scene`: Brush units are in meters. Zooming in and out in the viewport will make the brush grow/shrink relative to the view, but stay the same size relative to the scene.
The default radius unit is `Scene`, which is what GPv2 did.
The "2D Animation Template" was updated to disable using the radius in the unified paint settings. This means that using the template by default will use the brush radius, which also mimics the behavior of GPv2.
The user can change the radius brush unit from the `Advanced` panel.
Pull Request: https://projects.blender.org/blender/blender/pulls/120257
5d7e785fdd