Resize raster icons to their physical draw size

[anthony-firn]

Summary

Resize raster icons to their physical draw size in widget::icon.

This addresses the raster half of #565 without adding any surface-specific logic.

Why

Small raster app icons are currently filtered directly from their source image at draw time. On panel-sized surfaces that leaves some icons visibly rough and prone to shimmer.

Resizing once at the requested physical size produces a cleaner source image for the existing draw path, while keeping the change in the shared icon widget.

What changed

• add a cached raster resize path in widget::icon
• resize with premultiplied-alpha Lanczos filtering at the physical draw size
• reuse the resized handle from both Icon::view and icon::draw
• leave the SVG path unchanged for follow-up work

Validation

• cargo check -p libcosmic
• cargo test -p libcosmic lanczos_resize_produces_requested_dimensions --lib
• cargo test -p libcosmic premultiplied_resize_avoids_transparent_color_bleed --lib
• rebuilt cosmic-app-library against the patched libcosmic tree and verified smoother small raster app icons in a live COSMIC session

[mmstick]

iced maintains its own image cache for the renderer and performs resizing itself, so this would duplicate that work.

[mmstick]

@anthony-firn Can this be done in the iced renderer instead?

[anthony-firn]

Thanks, that makes sense. I moved the follow-up into the renderer instead so the change lives at the cache/upload layer instead of : pop-os/iced#318

This keeps the raster fix in the renderer and leaves untouched.

[anthony-firn]

Thanks, that makes sense. I moved the follow-up into the iced renderer instead so the change lives at the cache/upload layer instead of the libcosmic icon widget: pop-os/iced#318

This keeps the raster fix in the renderer and leaves libcosmic untouched.

[anthony-firn]

Closing this one in favor of the renderer-side follow-up at pop-os/iced#318, per the review feedback here.