When joined border width is zero width, then the midpoint
of the joined corner is no longer need to be computed
anymore. Just set the mid point to be the endpoint of the
corner.
The refactor of the border painting mainly to handle:
1. Single border with minor border radius.
2. Different border widths and border colors joined situations.
This refactor only apply to solid border.
The main differece is to use Path.fill to paint each border,
not fill_rect anymore. There's a special case need to consider.
The Path.fill will leave shared edge blank between two borders.
To handle this, we decide to combine the borders with same color
to paint together.
* A border-radius + a border on a <img> tag
- The border-radius on the <img> should shrink to line up with
the border.
* A border-radius + a border on a div with overflow: hidden
- The clipping border-radius should shrink (same as the image).
This adds test pages for border-radius, CSS custom properties and
flexboxes to the default page in the Browser.
I used those files to develop said features and they can be of use
when debugging in the future or just to showcase those features.
