Added roundRect support

[fgerodez]

Hi,

This is a pull request that exposes the new roundRect method.

As it is not currently supported on firefox I was wondering if we should add a warning in the usage section of the readme to link to a polyfill such as this one

[shamansir]

Could it be considered to use the type system for specifying the points? We lose the CSS flexibility in this case, but gain some more clarity, I think. For example: type alias Radii = { topLeft : Float, topRight : Float, bottomLeft : Float, bottomRight : Float }.

Optionally, some helpers could be provided, like : allCorners : Float -> Radii.

Or, make Radii a sum type:

type Radii
    = AllCorners Float
    | DiagonalCorners { topLeft : Float, bottomRight : Float }
    | FourCornersByThreeValues  { topLeft : Float, topRightBottomLeft : Float, bottomRight : Float }
    | FourCorners { topLeft : Float, topRight : Float, bottomLeft : Float, bottomRight : Float }
    
-- plus, some helpers as well

Both variants could always translate to four-valued list.

[mbylstra]

@joakin Would you consider approving this PR? Although I think @shamansir's suggestions are good, I also think the list of float's API in this PR is totally adequate, not a deal breaker - not something that needs to hold up the PR for several more years.

[mbylstra]

I have tested this branch locally by linking in the elm.json "src" - it is working well

[joakin]

@mbylstra sure, I'll have a look in the next two weeks, I'm on vacation without a computer right now.

The PR looks good but I'd like to specify the radii in Canvas.elm in a more type safe way like @shamansir suggested. Maybe just having the radii be a record of { topLeft, topRight, bottomLeft, bottomRight } would work fine, no need to over complicate the API.

Another option is to have roundRect take a single Float for all radii which is the most common case, and have a nother roundRectCustom or something like that that takes the record with the four corners.

[mbylstra]

Sound good! Please enjoy your vacation and take a break from coding 😄