[Uno] Default Opaque to false for SKXamlCanvas on MacOS and iOS

[roubachof]

Description of Change

See canvas default opacity to none on mac and iOS to align on other platforms.

Bugs Fixed

API Changes

None.

Behavioral Changes

By default, canvas will be transparent instead of black.

Required skia PR

None.

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Merged related skia PRs
• Changes adhere to coding standard
• Updated documentation

[mattleibow]

This is actually incorrect for macOS. There is no Opaque property, but rather IsOpaque. However, this other property is read only and according to the docs you should override the property to return the value you want:

The default value of this property is false to reflect the fact that views do no drawing by default. Subclasses can override this property and return true to indicate that the view completely covers its frame rectangle with opaque content. Doing so can improve performance during drawing operations by eliminating the need to render content behind the view.
https://developer.apple.com/documentation/appkit/nsview/1483558-isopaque

[mattleibow]

Fixed in #2477

[roubachof]

oops sorry about that ! I thought I tested it :/