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.
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