pdf-page: Fix image decoding correctness bugs

[Velli20]

• Use color_space.num_color_components() instead of inferring component count from raw data length, which was wrong for non-8bpc images
• Add explicit UnsupportedBitsPerComponent error for BitsPerComponent != 8 instead of silently producing garbage output
• Fix CMYK images: 4-component arm in to_rgba now converts CMYK→RGBA instead of treating the K channel as alpha
• Fix Indexed color space: expand palette indices at parse time via expand_indexed(), and store the base color space (not the Indexed wrapper). Keeping the Indexed wrapper caused resolve_image_data in the canvas layer to re-expand already-decoded pixel data, producing a buffer too small for the declared RGBA8888 format and causing Skia to fail with "failed to create raster image from data (NxN)"
• Rename PixelFormat::Alpha8 → Gray8 (it was already mapped to Skia's Gray8 color type; the name was just wrong)
• Fix fallback arm in to_rgba: use chunks_exact(num_color_components) to stay within each pixel's bytes instead of reading across pixel boundaries
• Add ZeroImageDimensions and ZeroColorComponents error variants; remove dead #[from] on Box in SMaskReadError