RawImage and its RenderImage render object both delegate their painting to paintImage, which already accepts a blendMode parameter that controls the Paint.blendMode used when compositing the image onto the canvas. Neither widget exposed this parameter though, so the blend mode was always stuck on the default BlendMode.srcOver. That made it awkward to use RawImage for effects like crossfading where BlendMode.plus is the natural fit, even though the underlying painting code fully supports it.

This PR threads a new blendMode parameter through RawImage, RenderImage, and the paintImage call. The default value is BlendMode.srcOver, matching the existing paintImage default, so behavior is unchanged for every existing caller. The implementation follows the same pattern used by filterQuality and other non-nullable enum properties on RenderImage (constructor argument, private field with getter/setter that calls markNeedsPaint, forwarded in paint, and reported via debugFillProperties).

Tests cover both the default value and a custom value being forwarded through to RenderImage from RawImage, plus the same on RenderImage directly.

Fixes #145614

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.