feat: add transform-skew-x/y support

[NLazyCat]

Add transform-skew-x and transform-skew-y properties to Slint, enabling CSS-like skew/oblique transforms on any element.

Closes the missing SkewTransform from the WinUI → Slint mapping.

Changes:

• Runtime: Added transform_skew_x/transform_skew_y properties to the Transform item and skew() method to the ItemRenderer trait
• Skia renderer: Implemented via canvas.skew() + euclid pre_skew
• FemtoVG renderer: Implemented via skew_x()/skew_y() with clip bounding box recomputation
• Software renderer: Stub (tracked in Path: fix panic when accessing properties on sub commands #6068, same as rotate/scale)
• Compiler: Registered transform-skew-x/transform-skew-y as reserved transform properties (typeregister, builtins, lowering, debug hooks)
• Coordinate mapping: Composed skew into children_transform() for correct input event handling

Usage:

Rectangle {
transform-skew-x: 20deg;
transform-skew-y: -10deg;
}

Testing:

• Added screenshot test (tests/screenshots/cases/transforms/skew.slint)
• All 3 backends pass: software, software_embed_assets, skia

[ogoffart]

Thanks for the PR. What is the use case for this feature?

[NLazyCat]

I'm doing a mechanical migration of a WinUI3 component library to Slint. WinUI3 has SkewTransform (AngleX/AngleY) and CompositeTransform (which includes SkewX/SkewY) — Slint currently has no equivalent for these, so transform-skew-x/y is needed to unblock the migration path.