Fix greyscale colormapping being not gamma-corrected (#2456)

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### What

Fix `colormap_srgb` outputting linear greyscale map instead of srgb
greyscale map.

Note how this method is documented as:
`/// Returns a gamma-space sRGB in 0-1 range.`
but then a few lines below outputted `Vec3(t)` directly, i.e. a linear
scale.

This became visually apparent when you put a 3 channel image (no color
map!) next to a single channel image (greyscale color map applied) next
to each other.

Before:
<img width="402" alt="image"
src="https://github.com/rerun-io/rerun/assets/1220815/65e0feaa-2443-483b-9b6f-424e6b22c0d7">

After:
<img width="411" alt="image"
src="https://github.com/rerun-io/rerun/assets/1220815/da1ba05e-7343-4700-a218-c5bb240870cb">


(this is not what the sample usually looks like, I made both images take
the value 128, they are different by default)


### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)

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PR Build Summary: https://build.rerun.io/pr/2456

<!-- pr-link-docs:start -->
Docs preview: https://rerun.io/preview/1c35935/docs
Examples preview: https://rerun.io/preview/1c35935/examples
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crates/re_renderer/shader/colormap.wgsl

@@ -15,7 +15,7 @@ const COLORMAP_VIRIDIS:   u32 = 6u;
 fn colormap_srgb(which: u32, t_unsaturated: f32) -> Vec3 {
     let t = saturate(t_unsaturated);
     if which == COLORMAP_GRAYSCALE {
-        return Vec3(t);
+        return srgb_from_linear(Vec3(t));
     } else if which == COLORMAP_INFERNO {
         return colormap_inferno_srgb(t);
     } else if which == COLORMAP_MAGMA {