[Enhancement] Thinned Caps Brush Rounding Mode

[athyfr]

Feature/enhancement description:

This new brush rounding mode would minimize the aliasing at the caps.

From godot-antialiased-line2d's README:

"Due to how the texture is tiled, Line2D endings will not be antialiased. Only the long sides are antialiased. In practice, this still looks quite good most of the time. You can adjust the caps or use variable width to make aliasing on the ends less noticeable."

I propose that a new brush rounding mode is added that thins the caps to 0 thickness, giving a bevelled look, and minimizing the aliasing.