[Build] Separate Resizetizer image processing from platform injection and add extensibility targets

[Redth]

Note

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Description

Fixes #34222
Part of #34099

This PR separates the Resizetizer's ResizetizeImages target into two distinct concerns and adds official extensibility points for custom platform backends.

Changes

1. Split ResizetizeImages into processing + injection targets

• ResizetizeImages now only handles platform-agnostic image processing (validate, resize, convert, collect results, stamp file)
• _ResizetizeInjectPlatformItems is a new target (AfterTargets="ResizetizeImages") that handles platform-specific output injection (BundleResource, LibraryResourceDirectories, ContentWithTargetPath, etc.)

2. Add extensibility hook targets with DependsOnTargets properties

Target	Property	Runs After
_ResizetizerAfterImageProcessing	ResizetizerAfterImageProcessingTargets	ResizetizeImages, before _ResizetizeInjectPlatformItems
_ResizetizerAfterFontProcessing	ResizetizerAfterFontProcessingTargets	ProcessMauiFonts
_ResizetizerAfterAllProcessing	ResizetizerAfterAllProcessingTargets	ResizetizeImages, ProcessMauiFonts, ProcessMauiAssets
_ResizetizeInjectPlatformItems	ResizetizeInjectPlatformItemsTargets	ResizetizeImages

Custom backends can register their targets via properties:

<PropertyGroup>
    <ResizetizerAfterImageProcessingTargets>
        $(ResizetizerAfterImageProcessingTargets);
        _MyCustomPlatformBundleImages
    </ResizetizerAfterImageProcessingTargets>
</PropertyGroup>

3. Optional custom platform targets import

Backends can set ResizetizerCustomPlatformTargets to import a custom targets file:

<PropertyGroup>
    <ResizetizerCustomPlatformTargets>$(MSBuildThisFileDirectory)Resizetizer.MyPlatform.targets</ResizetizerCustomPlatformTargets>
</PropertyGroup>

Impact

• Zero breaking changes — pure refactor; built-in platforms behave identically
• Official extension points — custom backends can reliably hook in without depending on internals
• All 587 Resizetizer unit tests pass

[Copilot]

Pull request overview

This PR refactors the Resizetizer build pipeline to separate platform-agnostic image processing from platform-specific item injection, and introduces new MSBuild hook targets/properties intended to make custom platform backends extensible without patching internal targets.

Changes:

• Adds hook targets for “after font processing”, “after image processing (before injection)”, and “after all processing”.
• Splits platform-specific output injection into a new _ResizetizeInjectPlatformItems target that runs after ResizetizeImages.
• Adds an optional ResizetizerCustomPlatformTargets import for backend-provided targets.

[Copilot]

_ResizetizerAfterAllProcessing is intended to run after all resource processing, but AfterTargets="ResizetizeImages;ProcessMauiFonts;ProcessMauiAssets" will schedule it after whichever of those targets runs first (the target will only execute once). Consider introducing an aggregate target that DependsOnTargets="ResizetizeImages;ProcessMauiFonts;ProcessMauiAssets" and then run _ResizetizerAfterAllProcessing after that aggregate, so the hook reliably runs after the full pipeline.

Suggested change

	<!-- Extensibility target: runs after all resource processing is complete.
	Custom backends can register targets via the ResizetizerAfterAllProcessingTargets property. -->
	<Target Name="_ResizetizerAfterAllProcessing"
	AfterTargets="ResizetizeImages;ProcessMauiFonts;ProcessMauiAssets"
	DependsOnTargets="$(ResizetizerAfterAllProcessingTargets)" />
	<!-- Aggregate target to ensure all resource processing has completed. -->
	<Target Name="_ResizetizerAllResourceProcessing"
	DependsOnTargets="ResizetizeImages;ProcessMauiFonts;ProcessMauiAssets" />
	<!-- Extensibility target: runs after all resource processing is complete.
	Custom backends can register targets via the ResizetizerAfterAllProcessingTargets property. -->
	<Target Name="_ResizetizerAfterAllProcessing"
	AfterTargets="_ResizetizerAllResourceProcessing"
	DependsOnTargets="$(ResizetizerAfterAllProcessingTargets)" />

[Copilot]

The new extensibility properties use the Resizetizer...Targets prefix (e.g., ResizetizerAfterImageProcessingTargets), but this target uses ResizetizeInjectPlatformItemsTargets (missing the 'r'). For a new public extension point, please align the property name with the other extensibility properties to avoid confusing custom backends and to match the target’s purpose.

Suggested change

	DependsOnTargets="$(ResizetizeInjectPlatformItemsTargets)">
	DependsOnTargets="$(ResizetizerInjectPlatformItemsTargets)">