Move PNG_* constants to constructor, clean up toBuffer API

[zbjornson]

Proposal for a breaking change in 2.0: move all the PNG_* constants from the Canvas prototype to be static members of the Canvas constructor. That is, this doc example:

var buf2 = canvas.toBuffer(undefined, 3, canvas.PNG_FILTER_NONE);

would become

var buf2 = canvas.toBuffer(undefined, 3, Canvas.PNG_FILTER_NONE);

The constants were added in 14d9f58. @kkoopa you added these -- do they need to be on the prototype?

[kkoopa]

I do not believe that they need to be on the prototype. The third argument was added in a strange backwards-compatible way, and you pass undefined first only in this case, so you can consider cleaning it up now.

…

On July 1, 2017 12:59:33 AM GMT+03:00, Zach Bjornson ***@***.***> wrote: Proposal for a breaking change in 2.0: move all the `PNG_*` constants from the Canvas prototype to be static members of the Canvas constructor. That is, this doc example: ```js var buf2 = canvas.toBuffer(undefined, 3, canvas.PNG_FILTER_NONE); ``` would become ```js var buf2 = canvas.toBuffer(undefined, 3, Canvas.PNG_FILTER_NONE); ``` The constants were added in 14d9f58. @kkoopa you added these -- do they need to be on the prototype?

[zbjornson]

I think this would be the wrong direction after all. If the Canvas spec ever gets an equivalent property, it won't be on the Canvas constructor, because there is no Canvas constructor. :)

Fine as-is methinks.