Brush tool/eraser UI improvement for semantic segmentation

**Is your feature request related to a problem? Please describe.**

Hi, thanks for the amazing project!
When using the brush and eraser tools for semantic segmentation labeling, there's no visual preview of the brush size after adjusting it. Users can only see the actual size of the brush after clicking on the canvas, which makes it a little difficult to choose the precise size of it.

**Describe the solution you'd like**

I think a preview of the brush/eraser size in the circular cursor would help pick the right size.
So when adjusting the size, the brush also updates dynamically.

**Describe alternatives you've considered**

The only alternative I can think of is making the size of the circular cursor proportional to the brush size.

**Additional context**
current UI:

improved UI with preview on cursor: