Improve gimp_gegl_mask_combine_ellipse_rect() -- the funciton
responsible for rendering ellipse/rounded-rectangle selections.

Most notably, this commit significantly improves the function's
performance, by identifying whole tiles, whole rows, or parts of a
row, that are fully inside, or fully outside, the ellipse, and
filling them in bulk, instead of calculating the anti-aliasing
value at each pixel, which is now only done along the
circumference.

This commit also improves anti-aliasing, by more accurately
approximating the distance from a pixel to the ellipse, and by
normalizing the distance according to the pixel's cross-section
length in the direction of the said point.  In particular, we
guarantee that pixels that are fully inside/outside the ellipse
have a value of 1/0, respectively, facilitating the aforementioned
optimization.

Additionally, this commit fixes various edge cases where several
primitives coincide at a single pixel (in the rounded-rectangle
case), adds support for CHANNEL_OP_INTERSECT, and parallelizes
processing.