Optimize Mesh::add_rect_with_uv

[valadaptive]

• I have followed the instructions in the PR template

While optimizing #7431, I noticed that Mesh::add_rect_with_uv was taking quite a bit of time. Using extend_with_slice results in much better codegen (fewer branches) and inlining the function makes things faster still. The same applies to Mesh::add_triangle.

Current main:

text_layout_uncached    time:   [65.988 µs 66.042 µs 66.108 µs]
                        change: [+0.1351% +0.3381% +0.5863%] (p = 0.00 < 0.05)

With these optimizations:

text_layout_uncached    time:   [59.996 µs 60.104 µs 60.269 µs]
                        change: [−9.8641% −9.6530% −9.4583%] (p = 0.00 < 0.05)
                        Performance has improved.

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7511-optimize-add-rect
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[lucasmerlin]

Nice, great find!