Fix findCubicRoots for cases where coefficients are close to zero

The equation is a*x^3 + b*x^2 + c*x + d = 0.
Previously, we would divide by zero if a = ~0.
This change also makes sure that we return zero no roots in the case
where a = ~0 and c*c - 4*b*d < 0, and the case where a = b = c = ~0.
Finally, we return 0 or 1 if we're close enough to assume that it could
be a numerical error.

This change also adds tests for the above cases.

Change-Id: I426d2fc6175b3aff6fe099845bf63d433c158536
Co-authored-by: Christian Strømme <christian.stromme@qt.io>
Reviewed-by: Christian Stromme <christian.stromme@qt.io>

0 files changed, 0 insertions, 0 deletions