feat(imaging): add pure-Rust Skia backend (#112)

crates/libremetaverse-imaging-skia/tests/fixtures/README.md

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+# Rust codec fixtures
+
+The tests decode these hexadecimal files into their exact checked-in bytes; no
+image is generated while tests run. Unless noted below, fixtures come from the
+BSD-3-Clause Skia test resources vendored by `skia-bindings 0.99.0`.
+
+| Fixture | SHA-256 |
+| --- | --- |
+| `alpha.webp` | `8b09993d8ffc9a9ec4e7f5f26a1c126cf265c37b0d63c0c5be2506e20dbc6fea` |
+| `animated.webp` | `292753f066add623af1e30bbeeca58f15b3d6d1052e12b13e30f21f8d3c14505` |
+| `baseline.jpg` | `4e45048dbe3078943eb364221a9757df9b9845b80e795ccdc944f1c18b5e221b` |
+| `bottom-up.bmp` | `d9cc5fff9e8e351727ad4519236ae46eaf37f8007dad5cdb8619fcd67ec68903` |
+| `gray-alpha.png` | `377a67510b4ccc74cdcce14847b02035e75ccab36e9d8e00586c4c24d1ae385a` |
+| `gray.png` | `85b19fcda452d6cbf02f570c353cfa9e47c29237f2fc8f0114deb6cd0b23f97a` |
+| `interlaced-palette.png` | `6283a61b1f7344c0cfa741076d6aed4a16a6bc5582e81db8dd7374a34230b0df` |
+| `palette-offset.gif` | `32d3409a79b75614ef385354d54f148f08856f8e9ce0d55054582ff7f40f4461` |
+| `progressive.jpg` | `d07a9815dc792543c16b796ec7729c19d6a29e7d9b4d0bbab50f4ef210be7767` |
+| `rgb.png` | `ecea1ef4a001a999ce9b82d18abf47401eb92998708b306b6c9a8ac3a6a9a2c8` |
+| `rgba.png` | `756a03364c02e3c9f85d6f4029eb3cef2488c081dc537d46e102bebcb9e02732` |
+| `straight-alpha.png` | `5a647404fa5289da777b1bb9e1dcfe20027262f992a214f38efaecc3da0e08a4` |
+
+`lossy-red.webp` was produced once with FFmpeg 5.1.8/libwebp from a solid
+8x8 red source (`-c:v libwebp -lossless 0 -quality 80`) and has SHA-256
+`7e3aaf1a442b6e69f7706ee5afe0aeacb2d096e384dbb63e792421fe2449cfe8`.
+
+The remaining tiny BMP, ICO, WBMP, and lossless RGB WebP files are original
+MetaCrate byte-level fixtures. Their reviewed bytes are intentionally readable
+in the `.hex` files and exercise header fields, row layout, masks, multi-byte
+integers, and multi-entry selection directly.