Align `Color32` to 4 bytes (#7318)

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It could be useful when you want to preprocess an image using SIMD
instructions without extra copying to convert it to a texture.

Possibly we might add it as a feature.

* Closes <https://github.com/emilk/egui/issues/7292>
* [x] I have followed the instructions in the PR template

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/ecolor/src/color32.rs

@@ -24,6 +24,7 @@ use crate::{Rgba, fast_round, linear_f32_from_linear_u8};
 ///
 /// An `alpha=0` means the color is to be treated as an additive color.
 #[repr(C)]
+#[repr(align(4))]
 #[derive(Clone, Copy, Default, Eq, Hash, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 #[cfg_attr(feature = "bytemuck", derive(bytemuck::Pod, bytemuck::Zeroable))]