Refactor: Remove image crate dependency from ribir_painter

[M-Adoo]

Goal

PixelImage in ribir_painter should only store raw pixel data. Image format handling (PNG, JPEG) should be moved elsewhere.

This will make the painter crate more focused and reduce dependencies.

Current State

ribir_painter currently has optional image crate dependency with png and jpeg features that provide:

• PixelImage::from_png() / PixelImage::from_jpeg()
• PixelImage::write_as_png() / PixelImage::write_as_jpeg()
• PixelImage::parse_img()

Blockers

1. Font Raster Glyphs

File: painter/src/text/font_db.rs:371

glyph_raster_image() parses PNG glyph images embedded in fonts. Need to decide:

• Keep as internal implementation
• Move to core
• Drop support

2. Dev-Helper Tests

File: dev-helper/src/image_test.rs

Uses write_as_png() / from_png() for test image comparison. Solution TBD.

3. Examples

Files: examples/messages, examples/storybook, examples/pomodoro

Currently use PixelImage::from_png(include_bytes!(...)).

Solution: Migrate to use Image widget via asset! macro.

4. GPU/Core Tests

Files: gpu/src/gpu_backend.rs, core/src/builtin_widgets/backdrop_filter.rs

Load test PNG files using PixelImage::from_png().

Solution: Migrate to use Image widget or core utilities.

Migration Plan

1. Migrate examples to use Image with asset! macro
2. Address dev-helper test image handling (solution TBD)
3. Decide on font raster glyph handling
4. Remove image dependency and png/jpeg features from painter