Fix comment style: add space after // (#8333)

## Summary

- [x] I have followed the instructions in the PR template

Fixes comment-style violations of the `CONTRIBUTING.md` code style rule:

> `// Comment like this.` and not `//like this`

### Files changed

- `crates/epaint/src/shapes/bezier_shape.rs`: `//temporary solution` →
`// temporary solution`, and 22× `//add the start point` → `// add the
start point`
- `crates/egui/src/animation_manager.rs`: `//start new animation…` → `//
start new animation…`
- `crates/eframe/src/web/web_painter_wgpu.rs`:
`//create_new.instance_descriptor…` → `//
create_new.instance_descriptor…`

No code logic changes — only adding the missing space after `//` in
inline and standalone comments.

## Test plan

- [x] `cargo fmt --check`
- [x] `cargo clippy -p epaint -p egui`
- [x] `cargo test -p epaint --lib`

crates/eframe/src/web/web_painter_wgpu.rs

@@ -90,7 +90,7 @@ impl WebPainterWgpu {
             && create_new.display_handle.is_none()
         {
             // Force WebGL, useful for quick & dirty testing:
-            //create_new.instance_descriptor.backends = wgpu::Backends::GL;
+            // create_new.instance_descriptor.backends = wgpu::Backends::GL;
             create_new.display_handle = Some(Box::new(WebDisplay));
         }