Make it so that we can use Web.DOM.Document createElement with the purescript-canvas library. Useful for double buffering.

[thanacles]

I would like to create a canvas that I could use for double buffering. I feel like this is a common use case for canvases and thus would be useful to others as well. So I add a createCanvas function to the library in this PR.

Issue: #75

[hdgarrood]

This is of course useful alongside these functions, but the purescript-web libraries are intended to be low level libraries that conform relatively closely to the relevant web APIs - and so this function should really live in web-dom, since it's part of the Document API. Specifically, see:

• https://pursuit.purescript.org/packages/purescript-web-dom/4.0.1/docs/Web.DOM.Document#v:createElement
• https://pursuit.purescript.org/packages/purescript-web-html/2.3.0/docs/Web.HTML.HTMLCanvasElement#v:fromElement

My worry with adding this here is that if we start adding functions because they're useful, it'll make more space for more opinionated decisions, which can increase the maintenance burden quite a bit.

[hdgarrood]

I suppose this library should arguably import HTMLCanvasElement instead of defining its own type, along the lines of what's discussed in #61.

[thanacles]

@hdgarrood Makes sense. Do you think there is a way to switch this library to use HTMLCanvasElement and stay reverse compatible? Or do you think, we should just bite the bullet and make a breaking change?

[hdgarrood]

It should be possible to replace the CanvasElement declaration with

type CanvasElement = HTMLCanvasElement

which I think would make it mostly non-breaking, although you could potentially still have issues if you're trying to declare type class instances for both CanvasElement and HTMLCanvasElement.

[thanacles]

How about something like this. There is more refactoring that can be done, mainly to do with reusing functions in the other libraries instead of duplicating. But, I feel like this change is disruptive enough, and refactoring should be delegated to the future. I want something like this change for a project I'm working on because I want to use double buffering.

[hdgarrood]

I would actually prefer to do this refactoring in one go (i.e. the entire task of #61 - splitting off the Context2D interface to a new web-context2d library) than spreading it out over multiple releases. I would recommend that you define coercions in your project as a stopgap, like this:

toHTMLCanvasElement :: CanvasElement -> HTMLCanvasElement
toHTMLCanvasElement = unsafeCoerce

or indeed just include the FFI wrapper for document.createElement("canvas") that you had in the first version of this PR locally in your project.

[thanacles]

OK, will do. :(