Support StrokeStyle

[sammykim]

I modified some codes in azure to support StrokeStyle like cap-style, join-style and some others. But there are some problems.
First of them, It doesn't look the join-style works well. I tested it join-style bits beside cap-style in C. It looks 21(cap,join) as hex in C. So as you can see in code, I implemented set method in azure_hl. I think the bit is set properly but the only join_style isn't applied that I want.(I tested CAP_BUTT and CAP_ROUND. Both of them work fine. )
Second, I will send PR about border-style after this is merged. I can render DashedBorderStyle but can't render DottedBorderStyle as I expect. Because I supposed servo and firefox uses same graphic engine. So I refered to firefox code(link : http://mxr.mozilla.org/mozilla-central/source/layout/base/nsCSSRenderingBorders.cpp#1080) to render them properly specially for DashPattern and cap, join style.
If you can catch what thoes problem are, let me know that I will fix it before it's merged. But I think second one is you might catch after I send main codes for border-style.

[sammykim]

As @sanxiyn commented , I fixed it. I set the bits as same as StrokeOptions initial values in 2D.h
JOIN_MITER_OR_BEVEL is 3_u32. CAP_BUTT is 0_u32. Those are in azure.rs.

[metajack]

Looks good other than the magic number.

[sammykim]

Addressed a comment.
r? @metajack. It it better to sqaush last commit?

[metajack]

Yes, please squash and I'll r+ and merge.

[sammykim]

Thanks, I squashed them.

[metajack]

r+