Support arbitrary angle ranges in conic-gradient

[task-jp]

Normalize conic gradient stop positions to [0,1] range using modulo wrapping, enabling support for angle ranges outside 0-360 degrees (e.g., -90deg to 270deg or 360deg to 720deg).

Key changes:

• Separate duplicate positions with different colors to prevent flickering
• Interpolate boundary colors between min/max stops for seamless gradients
• Add boundary stops at 0.0 and 1.0 when needed

Tests cover negative angles, 360deg+ offsets, and full 360deg range with different start/end colors.

[ogoffart]

What's the motivation behind this.

It looks like this makes Slint different from what the CSS spec says.

[ogoffart]

I notice that the top right image now show some green between the red and white, while the code has it after the blue.
If I test the conic-gradient(red 0deg, white 180deg, blue 360deg, green 540deg); with CSS, i get the previous image without green. Not the new one.

[task-jp]

Thanks for your review.

What's the motivation behind this.

In https://snapshots.slint.dev/master/editor/index.html?load_url=https://raw.githubusercontent.com/slint-ui/slint/master/examples/speedometer/demo.slint, CarButton tries to rotate angles in @conic-gradients when it is pressed. To solve this, I thought it would be good to support any degrees.

this makes Slint different from what the CSS spec says.

Right. should we introduce the [ from [ <angle> | <zero> ] ]? for such usecase?

[ogoffart]

I see. I can achieve the desired effect with

@conic-gradient( #020414 stop4 - 1turn, #ff0000 stop5 - 1turn, #000000 stop6 - 1turn, #020414 stop1, #ff0000 stop2, #020414 stop3, #020414 stop4, #ff0000 stop5, #000000 stop6);

I added stop4 to stop6 back at the beginning with a - 1turn
So when some value are more than 360deg, they don't wrap around, but the value before that wraps around.

I would prefer keeping close to the CSS spec for gradients.