[patch] on the performance of KisCircleMaskGenerator::valueAt

[Geoffry Song]

I noticed that KisCircleMaskGenerator::valueAt(), which can easily be
called millions of times in a single stroke, has some rather
inefficient code (in particular, an unnecessary sqrt()), so I took the
liberty of optimizing the code a bit. This patch should somewhat
improve the performance of large brushes, especially the sketch brush
with a large diameter. No, it is not a large difference.

On my machine, the old code took about 26 nanoseconds per call, and
the new code takes 15 nanoseconds per call.

(Raw data: old code, time per call: 2.6188e-08, 2.6148e-08, 2.612e-08,
2.6688e-08, 2.668e-08, 2.6868e-08. New code: 1.4952e-08, 1.5532e-08,
1.4972e-08, 1.4964e-08, 1.4892e-08, 1.4868e-08, 1.4876e-08,
1.5144e-08, 1.4872e-08, 1.4872e-08, 1.4876e-08. Each sample is an
average over 2.5 billion calls)


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