Support blend mode and better handling of transparency group XObjects

[PsykerUdot]

This pull request introduces several improvements and refactorings, focusing on blend mode support and better handling of transparency group XObjects (transparency groups are not yet implemented though).

• Extended the SKPaintCache to cache paints based on blend mode, removed the separate antialiasing paint fields, and updated all usages to pass blend mode explicitly. This enables correct rendering with different blend modes and improves paint reuse.
• Transparency group XObject is rendered to separate Skia layer, as such XObjects are required to be rendered to parent layer as a whole. PR Make extended graphics states stacked too UglyToad/PdfPig#1246 is required to properly handle some cases (see this sample document).
• Removed hacky IsAnnotationBelowText logic, as Highlight annotation normally has explicit appearence stream and blend modes are supported now.
• [Breaking change] Made SkiaStreamProcessor no more responsible for background color. Modified the GetPageAsSKBitmap method to accept an optional background color, defaulting to white in tests and PNG export, and updated test helpers accordingly.
• createEmptyDiffImage method is fixed, so it doesn't dispose returned bitmap (this caused AccessViolationException when running tests).
• Reference images are updated, as few PDF are rendered more correctly now (at least comparing to other PDF renderers).
• Test sample is added.

[BobLd]

@PsykerUdot thanks a lot for the great work here!! this is a major contribution

I submitted a comment, keep me posted.

I will update PdfPig with the latest pre-release and check 68-1990-01_A_1.png again

Thanks again!