Even when the pts are restricted to 32K values, we can still overflow computing

Even when the pts are restricted to 32K values, we can still overflow computing
a fixed-point coefficient for quadratics. To avoid this, we bias these
coefficients, storing 1/2 of their actual value, and then apply the 2x unbias
in updateQuadratic().

Fixes http://code.google.com/p/chromium/issues/detail?id=140803
Review URL: https://codereview.appspot.com/6450099

git-svn-id: http://skia.googlecode.com/svn/trunk@4960 2bbb7eff-a529-9590-31e7-b0007b416f81