fix: getScreenshot white screen under hybrid composition (#175)

[endigo]

Summary

Fixes #175: getScreenshot returned a blank white image.

Hybrid composition (initExpensiveAndroidView) and hardware layers break drawing-cache / plain View.draw captures. This PR captures real PDF content instead:

• Android: Prefer PixelCopy of the attached platform view; fall back to a temporary LAYER_TYPE_SOFTWARE + View.draw (no drawing cache).
• iOS: Implement getScreenshot by rasterizing the PDFKit layer; if that is blank, draw the current PDFPage into the viewport.

No package version bump (coordinator releases 1.5.0-beta.N).

Test plan

• flutter pub get && dart format . && flutter analyze && flutter test — all passed
• Android ./gradlew testDebugUnitTest — all passed, including new FlutterPDFViewScreenshotTest
• Manual: load a PDF in the example app, call controller.getScreenshot(path), open PNG and confirm page content (not white) on Android and iOS

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[endigo]

Review results (worker review-release-prs-353-357)

Quality gates

Check	Result
flutter analyze	✅
flutter test	✅ 95 passed
Android unit tests	✅ including FlutterPDFViewScreenshotTest

Review notes

• Android: PixelCopy primary path + temporary LAYER_TYPE_SOFTWARE draw fallback (restores layer type after).
• iOS: layer render + current PDFPage draw fallback.
• Correct fix for hybrid composition blank captures.

Merge + release

Merged into migrate/kotlin-swift as 1.5.0-beta.6 (tag v1.5.0-beta.6). pub.dev publish may retry after rate-limit window.