Add the operation 0x0C to draw rounded rectangles with varying radii

[GPrimola]

This PR adds the operation 0x0C to draw rounded rectangle with variable radius corners, using the already existing functionality on NanoVG.
This operation will be used on Scenic.Script.draw_rounded_rectangle as described by rounded rectangle canvas specs.

Result:

Left: green filled rectangle with corner radius: 25, 50, 75, 100
Right: empty rectangle with purple stroke with corner radius: 100, 75, 50, 25

[boydm]

I love how simple this looks.

Could you please add a comment here with the equivalent call/s in the Canvas API?

[boydm]

I'll look at merging this in after PR#18. I'll want to do some testing to make sure I can get the same output in the remote driver.

[boydm]

I'd like thoughts from others on if adding this to 0.11 is too big a change or if it should be the first new thing in 0.12.

I do want 0.12 to go out pretty quickly, so am leaning that way.

[GPrimola]

HTML <canvas> spec for rounded rectangle

[axelson]

Sounds like a great candidate for 0.12 to me 👍

[GPrimola]

@ringlej @crertel would you mind to take a look at this PR too?

Many thanks!

[crertel]

Excited about this, may need to figure out how to make it work with Cairo too.

I'm going to go ahead and merge this into a new branch, draw_rrectv, so we can start getting it folded into Cairo after rebasing.