Fix tile mode used in SkShader for a TileBrush with None TileMode

[emmauss]

What does the pull request do?

Changes the SKShaderTileMode used for TileBrushes with None Tile from Clamp to Decal. This removes the clamping effect on tile brushes that do not fill the control's bounds, and makes tile brushes behave closer to Wpf's in that regard.

What is the current behavior?

When TileBrush is used with TileMode None and destination rect doesn't fill the control's bounds, the clamping effect causes smearing along the axes.

What is the updated/expected behavior with this PR?

Using Decal skia tile mode instead of Clamps, the smear is gone;

How was the solution implemented (if it's not obvious)?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0049006-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]