Add kernelUnitLength support to feDiffuseLighting, feSpecularLighting, and feConvolveMatrix

[edco]

Implements kernelUnitLength support to enable scale-independent lighting effects.
Fixes #840 if the SVG is updated to include kernelUnitLength:

<svg id="svg1" viewBox="0 0 200 200" xmlns="http://www.w3.org/2000/svg">
    <title>lighting-color=hsla (SVG 2)</title>
    <desc>Looks like alpha should be ignored.</desc>

    <radialGradient id="rg1" r="0.5">
        <stop offset="0.5" stop-color="white"/>
        <stop offset="1" stop-color="black" stop-opacity="0"/>
    </radialGradient>
    <filter id="filter1" color-interpolation-filters="sRGB">
        <feSpecularLighting in="SourceAlpha" lighting-color="hsla(120, 100%, 25%, 0.5)"
                            specularConstant="8" specularExponent="10"
                            kernelUnitLength="1">
            <fePointLight x="100" y="100" z="10"/>
        </feSpecularLighting>
    </filter>
    <rect id="rect1" x="20" y="20" width="160" height="160"
          fill="url(#rg1)" filter="url(#filter1)"/>

    <!-- image frame -->
    <rect id="frame" x="1" y="1" width="198" height="198" fill="none" stroke="black"/>
</svg>

(without kernelUnitLength in the SVG the current behaviour is not a bug, and remains unchanged)

• Parse kernelUnitLength in crates/usvg/src/parser/filter.rs for DiffuseLighting, SpecularLighting, and ConvolveMatrix
• Add kernel_unit_length field and accessors to usvg AST structs
• Write kernelUnitLength attribute in crates/usvg/src/writer.rs
• Implement resolution-independent differential step and bilinear sampling in resvg lighting and convolve_matrix filter primitives
• Add unit and integration tests for kernelUnitLength parsing and rendering

Authored mostly by Gemini 3.7 Flash in Google Antigravity IDE.