canvas renderer doesn't properly render interior rings in polygons

[tschaub]

The canvas renderer needs to change the canvas context's globalCompositeOperation to properly erase interior rings from polygons. See  this Canvas.js for one way to do this.

[tschaub]

holes in polygons

[crschmidt]

In safari, after applying this patch, I get something that looks better, but still 'not quite right'; it looks fine when I'm sketching, but as soon as the geometry is finalized, it has a 'half-fill' look to it.

I guess the fact that it looks okay while sketching isn't new; it looks like that's the case even without this change, so I guess that's not relevant. (But maybe helps to better understand a potential fix? Ohhhh, wait. The sketch handler is using the default SVG stuff on a separate layer, so that's why it works. Okay, so ignore that :)

Okay, so knowing that, I think this is just because we're not setting the fillOpacity to 1.0 when drawing the inner ring. Attaching a patch that does that, which seems to fix the behavior I'm seeing.

[tschaub]

I've incorporated the fix for this into the patch for #3207. Proper hit detection of polygons with holes requires better rendering of holes.

[tschaub]

In w/ r11850.