Fix rendering hashes, empty bounds, and script preservation

[CoffeeFlux]

Rendering hashes could miss movement within an unchanged bounding box and depend on the previous contents of the output buffer. Empty frames could also retain a previous visible rectangle.

• Hash each image’s position, dimensions, color, and complete bitmap rows, excluding stride padding and caller-owned output bytes.
• Clear empty-frame results while preserving libass’s cached visible result.
• Preserve Kerning, Language, and LayoutResX/Y in the Aegisub wrapper.
• Accept LF and CRLF ASS files, with or without a UTF-8 BOM, in the C++ example.

Validation: all three Meson tests pass on macOS arm64 with zlib enabled and disabled. Regression coverage includes reused buffers, unchanged-frame caching, empty scripts, movement within fixed bounds, equal-area drawings with different pixel arrangements, wrapper/native rendering parity, and all four newline/BOM combinations. The new C, wrapper, and newline tests fail against the corresponding original code.

CI: Windows x64, macOS arm64, macOS x86_64, Linux with system dependencies, and Linux without zlib all pass.

Hash values intentionally change with this correction; previously stored hashes should be regenerated.

Opus reviewed fix-rendering-inspection with the prompt: "Please review the fix-rendering-inspection branch. Don't use your code review skill." Its static review found no bugs in the fixes. It did not independently execute the tests; the local and CI results above provide runtime validation. Minor suggestions concerned comments/includes, release-time versioning, and pre-existing example error handling.