Fix screenspace_texture example shader compiler error in WebGPU (Browser)

[marlyx]

Objective

Fix the screenspace_texture example not working on the WebGPU examples page. Currently it fails with the following error in the browser console:

1 error(s) generated while compiling the shader:
:213:9 error: redeclaration of 'uv'
    let uv = coords_to_viewport_uv(position.xy, view.viewport);
        ^^

:211:14 note: 'uv' previously declared here
@location(2) uv: vec2<f32>,

Solution

Rename the shader variable uv to viewport_uv to prevent variable redeclaration error.

[mockersf]

naga will error on this on all backends on the next version: gfx-rs/naga#2342