fix rounded border

[Sharktheone]

Currently, in our render backend, border-radius doesn't work. It produces very incorrect results.

For that to be fixed though, we need #650 to also be fixed at least for the border-radius part

However, it can be fixed by mutating the default border-radius values. Not optimal, but it would work to resolve the issue.

[Sharktheone]

for anyone looking for pointers, the issue lives propably somewhere in here:

gosub-engine/crates/gosub_vello/src/border.rs

Line 130 in 8a01d75

match radius {