handle large conic strokes better

A stroked conic computes the outset quad's control point by
computing the intersection of the quad's endpoints. If the
the denominator used to compute the scale factor for the
control point is small, check to see if the numerator is also
small so that the division stays bounded.

Also clean up error returns and internal function calls to
simplify the code.

Additionally, remove comic max curvature (unimplemented) and call
extrema functions instead to handle cases where the conic is degenerate
or is a line.

R=reed@google.com, fmalita@chromium.org
BUG=skia:3843

Review URL: https://codereview.chromium.org/1144883003
6 files changed