Add function to draw a filled circle

[ldicker83]

NAS2D provides basic circle filling. Would like to extend this to include filled circles.

[DanRStevens]

Is this something we really need? There doesn't seem to be any pressing need for it. I mean, it's been open for almost 7 years with no progress, so it can't be all that important.

Only place I can think of that had circular elements, were the planets in OPHD, and that was handled with sprites.

Also, the initial description is a bit confusing. It mentions providing basic circle filling, and extending that to include filled circles. Do we even have the initial functionality listed? There is a drawCircle function, though that's really more about drawing a regular polygon rather than a circle. At best it can approximate a circle by using a large number of sides. It's not really super clear what this issue is really about.

If we're going to keep this open, maybe we need to specify the goal a little more clearly. Failing that, perhaps we should just close this as something we won't implement.

[cugone]

I thought I'd implemented this, re: comments mentioned #427 issue but for the life of me I can't find the code locally. It may have been purged since I built a new computer since then. Not a big deal to re-implement it if you want.

[ldicker83]

I would welcome the contribution!

[DanRStevens]

Helpful that we have a test project now to examine things visually.

Curious if the implementation was something fast like Bresenham's circle drawing algorithm, or something slow based on trig functions.

I suppose that shouldn't matter too much if there is currently no implementation.

[cugone]

Bresenham's algorithm is for rasterization, the GPU does this automatically. Since it's a circle, and as a first step of "get it working first" I'd use a triangle fan and matrix transform to scale/translate it. A second "more efficient" version can be done on the GPU but it's complex to get working.

That said, the base nas2d-core project doesn't even compile. See #1149.