Pixel format types and RGBA8 conversion #66

Define common pixel format types and conversion utilities in the image crate.

Requirements#

• Define Image struct: width, height, pixel data in RGBA8 format (Vec, 4 bytes per pixel)
• Conversion functions:
  • Grayscale (1 channel) → RGBA8
  • Grayscale + Alpha (2 channels) → RGBA8
  • RGB (3 channels) → RGBA8
  • RGBA (4 channels) → RGBA8 (identity/validation)
  • Indexed color (palette + indices) → RGBA8
• ImageError enum for decoding failures

Acceptance Criteria#

• All conversion functions produce correct RGBA8 output
• Unit tests for each conversion path
• Edge cases: zero-dimension images, empty pixel data
• No external dependencies, no unsafe

Context#

Common infrastructure used by PNG, JPEG, and GIF decoders. Should be implemented early so decoders can target a uniform output format.

Phase 7 — Image Decoders