feat: Composition brush

[zxbmmmmmmmmm]

This pull request implements composition brush types that conform to Avalonia's brush interfaces. It also introduces new interpolators for RelativePoint and RelativeScalar to support smooth brush animations, and adds a new interactive "Composition brush" demo to the CompositionPage in the ControlCatalog sample.

These changes collectively provide a robust demonstration and infrastructure for animated composition brushes in Avalonia, both in the UI sample and the underlying rendering system.

What does the pull request do?

Composition brush implementation:

• Added new composition brush classes (ServerCompositionBrush, ServerCompositionGradientBrush, etc.) that implement Avalonia's brush interfaces, including property change tracking and deserialization for gradient stops and spread methods.

Animation infrastructure improvements:

• Introduced RelativePointInterpolator and RelativeScalarInterpolator to enable smooth animation of RelativePoint and RelativeScalar properties, which are essential for animating gradient brush parameters.

Composition brush demo and animation support:

• Added a new "Composition brush" tab to CompositionPage.axaml, featuring interactive UI elements for creating and animating solid, linear, radial, and conic composition brushes with animated properties and gradient stops.

How was the solution implemented (if it's not obvious)?

#20150 (comment)

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None

Obsoletions / Deprecations

None

Fixed issues

fixed #20150

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0060477-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[kekekeks]

Should probably add ctors to FFI too, e. g.

Avalonia/src/Avalonia.Base/Rendering/Composition/Expressions/BuiltInExpressionFfi.cs

Lines 131 to 136 in 5198744

	"ColorRGB", (float a, float r, float g, float b) => Avalonia.Media.Color.FromArgb(
	(byte) MathUtilities.Clamp(a, 0, 255),
	(byte) MathUtilities.Clamp(r, 0, 255),
	(byte) MathUtilities.Clamp(g, 0, 255),
	(byte) MathUtilities.Clamp(b, 0, 255)
	)

Not sure how to handle RelativeUnit there, might need a separate VariantType and separate ffi functions as enum value "ctors".

[kekekeks]

@maxkatz6 I think we might want to mirror UWP API here rather than Avalonia one, at least for the sake of compatibility. This needs an API review

[kekekeks]

In general our RelativePoint system felt a bit cumbersome to use compared to MS MappingMode one. We could use XPF approach to map brushes (see MuceRadialGradientBrush).

[zxbmmmmmmmmm]

In general our RelativePoint system felt a bit cumbersome to use compared to MS MappingMode one.

If we handle RelativeUnit directly like I'm currently doing, we may have to override their operators such as + - * /, which may not be a good way

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0060559-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0060650-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]