add canvas api support

[josedonizetti]

There a few methods/properties missing. Which I will add later. But a lot can be done with the ones supported already.

[dominikh]

Thanks.

A number of stylistic issues first:

1. Please run gofmt on the code
2. Please write comments like // comment here, not //comment here
3. For the comments that act as headers (e.g. the one in line 1776), please leave a blank line after them so that godoc doesn't associate them with the functions that follow immediately.

4. Instead of writing

   if maxWidth == -1 {
      something()
   } else {
      somethingElse()
   }

   write

   if maxWidth == -1 {
      something()
      return
   } 
   somethingElse()

That's as far as global stylistic issues go. Notes to specific lines will follow as inline comments.

[dominikh]

Please don't let the amount of "complaints" discourage you; almost all of them are superficial and just style nitpicks. I didn't spot anything actually wrong :-)

[josedonizetti]

@dominikh Think I fixed all of them. And don't worry about giving feedback, is always good. I'm still learning go, and sometimes forget about the styling rules. Thanks! :)

[dominikh]

Merged as 78f0940, thank you. The only code change that I added is that I changed counterclockwise from int to bool. That didn't warrant another revision of this PR :)