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.