Allow most CGContext methods to be usable from a immutable reference and expose several more methods

[mehcode]

If this is too much to submit at once, please tell me. This is what I've got left to send in and I'm unsure where to break it up more.

---

This change is 

[jrmuizel]

Shouldn't these methods be taking a &mut? They are very much mutating the underlying object.

[mehcode]

I don't disagree with you. I don't think foreign_type! { supports generating a CGContextRefMut structure to use though. Several clearly mutating methods that exist already took &self so I was copying what was there. Properly adding _RefMut and &mut self sounds like a breaking change and a large endeavor.

---

As a data point, my use case here is:

My use case here is:

let context: *mut Object = msg_send![class!(NSGraphicsContext), currentContext];
let context: *mut c_void = msg_send![context, CGContext];

let context = CGContextRef::from_ptr(context as *mut _);

context.save();
// [...]
context.restore();

Before this PR I was using CGContext::wrap_under_ and mem::forget.

[jrmuizel]

Ok. Let's take this.
@bors-servo r+

[bors-servo]

📌 Commit 8052ba2 has been approved by jrmuizel

[bors-servo]

⌛ Testing commit 8052ba2 with merge c9616cc...

[bors-servo]

☀️ Test successful - status-travis
Approved by: jrmuizel
Pushing c9616cc to master...