Support Android API < 28

[adamglin0]

1. The initial code is copied from composeShadowPlus. (So I should add a license in the code file.)

2. If android sdk < 28, the content should be wrapped with AndroidView and the LayerType should be set to LAYER_TYPE_SOFTWARE.

3. compose-shadow should provide a wrapper component to wrap content like ComposeShadowPlus.

4. It seems that the shadow drawn by the composeShadowPlus solution is different in android 27 and android 28.

Summary by Sourcery

Enable backward-compatible soft shadows on pre-API 28 devices by introducing a software-rendered ComposeView wrapper and custom shadow modifier, and update the sample app accordingly.

New Features:

• Add softLayerShadow modifier to draw shadows using NativePaint.setShadowLayer in Compose.
• Introduce SoftLayerShadowContainer composable to wrap content in a software-rendered ComposeView on Android versions below API 28.
• Add spreadScale utility function for calculating shadow spread scaling.

Enhancements:

• Update the sample MainActivity to demonstrate the new softLayerShadow within the SoftLayerShadowContainer.

Build:

• Lower Android minSdk version from 28 to 21.

[adamglin0]

Just a trial, the effect image on Android 25.

[adamglin0]

API 34

[adamglin0]

No solution to support innerShadow in android sdk < 28

[adamglin0]

@sourcery-ai review

[sourcery-ai]

Reviewer's Guide

This PR adds support for Android API levels below 28 by introducing a software‐layer shadow solution: it lowers the min SDK, provides a SoftLayerShadowContainer that wraps Compose content in an AndroidView with LAYER_TYPE_SOFTWARE on older devices, implements a softLayerShadow modifier using drawWithCache and native setShadowLayer, and updates the sample app to demonstrate the new wrapper and modifier.

File-Level Changes

Change	Details	Files
Introduce SoftLayerShadowContainer to conditionally wrap content in an AndroidView with software layer for API<28	Add SoftLayerShadowContainer Composable Use AndroidView factory to set View.LAYER_TYPE_SOFTWARE Fallback to direct content for API>=28	compose-shadow/src/androidMain/kotlin/com/adamglin/composeshadow/SoftLayerShadowContainer.kt
Implement softLayerShadow modifier for drawing shadows via native setShadowLayer	Create softLayerShadow() extension using drawWithCache Compute radius, paint configuration, spread via utils.spreadScale Draw outline and shadow using canvas.withSave in onDrawBehind	compose-shadow/src/androidMain/kotlin/com/adamglin/composeshadow/softShadow.kt compose-shadow/src/commonMain/kotlin/com/adamglin/composeshadow/utils/spreadScale.kt
Update sample app to showcase the new shadow wrapper and modifier	Wrap SampleApp in SoftLayerShadowContainer Add Box with softLayerShadow and background to MainActivity Import SoftLayerShadowContainer and softLayerShadow in MainActivity	sample/compose-app/src/androidMain/kotlin/com/adamglin/composeshadow/app/MainActivity.kt
Lower minimum Android SDK and prepare license inclusion	Change androidMinSdk from 28 to 21 in libs.versions.toml Add placeholder to include license header in copied code	gradle/libs.versions.toml

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[sourcery-ai]

Hey @adamglin0 - I've reviewed your changes - here's some feedback:

• Please add the original composeShadowPlus license header to the new Kotlin source files to ensure proper attribution.
• Consider giving SoftLayerShadowContainer a modifier: Modifier parameter instead of always using fillMaxSize() so it can be sized flexibly by callers.
• In softLayerShadow, the call to shape.createOutline hard-codes LayoutDirection.Rtl; it should use the current layoutDirection from the Compose scope for correct mirroring behavior.

Here's what I looked at during the review

• 🟢 General issues: all looks good
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟢 Documentation: all looks good

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