Dashed borders should draw with a tight bounding rect.

[gw3583]

When dashes are drawn we draw vertices using the bounds of the corner rect. This results in a lot of overdraw for corners with a lot of dashes. This is generally not an issue since the corners are cached and the dash count / corner size is typically quite low.

However, in some Gecko reftests, the combination of the above with a software rasterizer, can make this take so long that it times out the reftest.

To fix it, we should use a tight bounding rect for the vertices around a dash in order to drastically reduce overdraw.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1510076 for more information.

[gw3583]

@nical fixed this.