Based on the spotlight section of https://learnopengl.com/Lighting/Light-casters.
Note that we need to use textureSampleLevel in the shader and sample from mip level 1.0 if we sample inside a conditional rather than clamp the intensity value.
See gfx-rs/wgpu-rs#912 and https://themaister.net/blog/2019/09/12/the-weird-world-of-shader-divergence-and-lod/ for more details.