Expose Skia Blend mode to Image control

[leocb]

Hi, I'm working on a UI that's based on a 3D render, one of the outputs from the the render is a shadow pass, which to look correct must be set to the "Multiply" blending mode, which is not currently easily available.

Looking around, it seems the Skia renderer supports this natively: doc link. So maybe it would be easy to expose this? Let me know if it's something I should be able to easily implement myself

How it looks today (no blending mode), Notice how the shadow make the rail look gray under the button:

How it should look (Multiply blend mode), Notice the correct shadow:

[timunie]

If you inherit control you can override OnRender and get Skia API. See Avalonia.Samples SnowflakesControl sample

[leocb]

Seems like Multiply is not available as a blendMode on the context, I'll see if I can add it myself and open a PR

[leocb]

I've added support for what I suggested in this discussion here: #17903