Smooth (Phong) shading when using `fill-extrusion-rounded-corner-distance`

User Story

As a designer, I'd like buildings to be rendered with smooth shading when using the fill-extrusion-rounded-corner-distance setting so that my users don't see individual polygons that make up the corner smoothing.

Rationale

The new fill-extrusion-rounded-corner-distance feature makes for very pleasing looking buildings... mostly! Right now it adds extra edges which seems to counteract the desired effect. This is traditionally fixed in computer graphics with Phong shading (not to be confused with the Phong reflection model) where the normals are interpolated instead of calculated as flat planes resulting in a smoother appearance that doesn't add any additional polygons. This would avoid the following flat look of building edges:

IMO this doesn't require an extra flag or anything on the developer's end. I'd default it on when using the rounded corner distance option.

Impact

• Nicer looking buildings with pretty much no rendering cost!
• If you don't do it, they'll just look like they do currently which is fine I suppose.

AI Assistance

NA