Fix WebGPU error in "ui_pipeline" by adding a flat interpolate attribute

[ameknite]

Objective

• Fix this error to be able to run UI examples in WebGPU

1 error(s) generated while compiling the shader:
:31:18 error: integral user-defined vertex outputs must have a flat interpolation attribute
    @location(3) mode: u32,
                 ^^^^

:36:1 note: while analyzing entry point 'vertex'
fn vertex(
^^

It was introduce in #8793

Solution

• Add @interpolate(flat) to the mode field

[alice-i-cecile]

A simple enough fix, and a good whitespace fix thrown in.

Sorry about the random ping stranger 😅 Next time @ameknite, you can use code blocks to suppress accidental pings like that :)

[ameknite]

@alice-i-cecile oh I didn't realize, fixed, thanks.

[mockersf]

This is part of the wgsl spec: https://www.w3.org/TR/WGSL/#interpolation

User-defined vertex outputs and fragment inputs of scalar or vector integer type must always be specified as @interpolate(flat).

If wgpu doesn't catch it on all stacks, it should be added there