perf(ui-egui): eliminate zero-filling allocation on canvas texture upload in color_image (#4504)

- Replace vec![Color32::TRANSPARENT; N] zero-initialization with direct single-copy bytemuck::cast_slice(&img.pixels).to_vec() in color_image.
- Eliminates redundant zero-filling of ~20.7 MB buffer on every canvas frame render on Retina displays, saving memory bandwidth and allocation overhead.
- Fallback preserves transparent image for undersized or mismatched pixel buffers.

Fixes #4504.

crates/ui-egui/src/canvas/raster.rs

@@ -5,12 +5,13 @@ use super::{Affine, CacheKey, VectorcraftApp, Xf};
 /// A raster whose bytes don't match its size gives an empty image of that size.
 pub(crate) fn color_image(img: &vectorcraft_render::Rendered) -> egui::ColorImage {
     let size = [img.width as usize, img.height as usize];
-    let mut pixels = vec![egui::Color32::TRANSPARENT; size[0] * size[1]];
-    let bytes: &mut [u8] = bytemuck::cast_slice_mut(&mut pixels);
-    if bytes.len() == img.pixels.len() {
-        bytes.copy_from_slice(&img.pixels);
+    let expected = size[0].saturating_mul(size[1]).saturating_mul(4);
+    if expected == img.pixels.len() {
+        let pixels: Vec<egui::Color32> = bytemuck::cast_slice(&img.pixels).to_vec();
+        egui::ColorImage::new(size, pixels)
+    } else {
+        egui::ColorImage::new(size, vec![egui::Color32::TRANSPARENT; size[0].saturating_mul(size[1])])
     }
-    egui::ColorImage::new(size, pixels)
 }
 
 /// The pixels of the `w`×`h` frame through `view` to render again for `doc`, when only the document