Fix overlapping polygon rendering

[kkaefer]

Overlapping polygons on the same layer are rendered with a winding fill right now (due to the stencil inverting). We can solve this by computing pairwise intersections of all overlapping polygons and then render those as well to invert the fill again.

[kkaefer]

http://commaexcess.com/articles/7/concave-polygon-triangulation-shortcut mentions that this technique "Can be extended to include nonzero rendering by utilizing backface culling."

[kkaefer]

We can't just use non-zero drawing because there are buildings that have holes. Instead, we need to first draw all outer polygon rings, then subtract all inner polygon rings.

[kkaefer]

With this code, we can draw both overlapping polygons and holes in polygons:

gl.disable(gl.BLEND);
gl.stencilMask(0xFF);
gl.enable(gl.STENCIL_TEST);
gl.colorMask(false, false, false, false);
gl.clear(gl.STENCIL_BUFFER_BIT);

// Draw old-style
// gl.stencilOp(gl.INVERT, gl.KEEP, gl.KEEP);
// gl.stencilFunc(gl.NEVER, 1, 0x1);
// draw(gl, 'TRIANGLE_FAN', shape, [0, 1, 1, 1]);


// Draw front facing triangles
gl.stencilOpSeparate(gl.FRONT, gl.INCR, gl.KEEP, gl.KEEP);
gl.stencilOpSeparate(gl.BACK, gl.KEEP, gl.KEEP, gl.KEEP);
gl.stencilFunc(gl.NEVER, 1, 0xFF);
draw(gl, 'TRIANGLE_FAN', shape, [0, 1, 1, 1]);

// Decrease back facing triangles
gl.stencilOpSeparate(gl.BACK,  gl.DECR, gl.KEEP, gl.KEEP);
gl.stencilOpSeparate(gl.FRONT, gl.KEEP, gl.KEEP, gl.KEEP);
draw(gl, 'TRIANGLE_FAN', shape, [0, 1, 1, 1])

// Draw filling rectangle
gl.stencilOp(gl.KEEP, gl.KEEP, gl.KEEP);
gl.colorMask(true, true, true, true);
gl.stencilFunc(gl.NOTEQUAL, 0x0, 0xff);
draw(gl, 'TRIANGLE_STRIP', fill, [0, 0, 1, 1]);

All shapes with a CCW winding order are filled, while all shapes with a CW winding order are subtracted. Note that this comes at the cost of an additional draw call. We also need to make sure that the winding order is absolutely correct. Currently, we generate TRIANGLES rather than TRIANGLE_FANs that all have the same (clockwise) winding order. It's also unclear whether using a triangle fan that is anchored at 0,0 generates more fragments to be shaded because of the larger inverted area.

[kkaefer]