Closes#22433
Before/After (macOS):

For some reason the non-blurred one seems much lower quality, so we may
need to tinker with the samples, or something else.

I'm unsure if this is a problem on Linux/in the Blade renderer, but
since no changes were made outside of the medal shaders we can probably
take this macOS-specific win for now.
Release Notes:
- gpui: Fixed an issue where shadows with a `blur_radius` of 0 would not
render.