canvas: Scale stroke width based on shape transform

[Herschel]

Render strokes with the proper width in the canvas backend:

• Stroke width gets scaled based on the shape's transform and line style scaling settings
• Minimum 1 pixel width (1 twip hairline strokes get scaled to 1 pixel)

TODO:

• Move the line scale calculation to core::shape_utils because other renderers will want it as well
• Handle complex fill styles in strokes (need to create a fresh CanvasGradient instance per draw)
• Only calculate line scales if necessary (slight optimization)

Fixes the line widths in #751 in the canvas backend.

[Toad06]

This is also greatly improving the Steppenwolf games. 👍

Master:

This PR (matches Flash):

[Herschel]

@Toad06 Could you give this another test? Thank you!

Unfortunately with the canvas API, there's no good way to handle gradient strokes -- the trick that I use for gradient fills can't be applied to strokes, because transforms end up distorting the stroke geometry (joins/endcaps). So this PR takes the middle ground of creating a fresh gradient each frame for strokes, with a "simple" transform (rotation/scale, no skew), which should be good enough in the majority of cases.

[Toad06]

This looks good to me, this is really a significant improvement. 😉

(By the way, while testing I did notice a couple of regressions affecting the canvas renderer. They are not related to this PR. I reported them in #7448 before realizing this was an issue you were aware of already 😛)