spase_strips: Implement some FilterFunction variants with ColorMatrix

[waywardmonkeys]

This hooks up the color-adjustment FilterFunction variants by lowering them to ColorMatrix primitives.

Filter::from_function now supports:

• Brightness
• Contrast
• Grayscale
• HueRotate
• Invert
• Opacity
• Saturate
• Sepia

Blur continues to lower to GaussianBlur.

The matrix formulas follow the CSS/SVG Filter Effects shorthand definitions. The implementation keeps the matrix constructors private, so this uses the existing FilterFunction -> FilterPrimitive -> PreparedFilter::ColorMatrix path without adding new public API.

Notes

• grayscale, saturate, and the reusable full grayscale matrix use the current sRGB luminance coefficients: 0.2126, 0.7152, 0.0722.
• hueRotate keeps the older rounded coefficients from the spec’s hue-rotate matrix instead of substituting the newer luminance constants.
• grayscale, invert, opacity, and sepia clamp their bounded amount values before lowering.
• saturate, brightness, and contrast allow values above 1.0, matching the filter-function behavior.

[waywardmonkeys]

This was done with the assistance of Codex (GPT 5.5, xhigh).

[waywardmonkeys]

This is a draft because it is layered on top of the changes to add ColorMatrix in #1623.

[waywardmonkeys]

Once we add ComponentTransfer, then some of these can migrate to using that ... like brightness, contrast, opacity, and invert.

[grebmeg]

I think this PR is quite obsolete now, and I’m curious what the blocker was for merging it. Should we close it for now and reimplement it on top of the latest main when needed?