Fix windows plugins search path
update openfx
Interpolation: add functions to derive and integrate
BUGS: curve editor
Engine: use consistent set/get method naming, and avoid misleading inversions (IsSignificant was the opposite of EvaluateOnChange)
OfxImageEffectInstance: turnOffUndoRedo() was removed
Curve: bug fix - even when there is one keyframes, it’s a curve: there may be tangents!
Knob.h: WARN_UNUSED_RETURN (and get one more warning)
Knob/Curve: implement derive()
Interpolation: implement derive_clamp instead of interpolate_and_clamp
INterpolation: add polynomial equation solvers
Interpolation: add integrate_clamp
File_Knob: remove buggy unused function
Knob::evaluateValueChange: fix bug introduced in d27f94b523e634698022ff08cdf142814322c516 (easy to find now that there are no more inverse values, like isSignificant and isInsignificant)
EffectInstance::evaluate: fix bug signaled by static analysis (knob may be NULL)
Project::stackEvaluateRequest: fix misnamed variable (wasBeginCalled actually meant that begin was called before entering the function, but was called within the function)
Interpolation: fix compilation, cart is part of C99 and C++11, but not ISO C++/C++98
Interpolation: fix compilation, cbrt is part of C99 and C++11, but not ISO C++/C++98
Travis CI: OFX plugins cannot use parallel build because they share objects
Travis CI: don't use MAKEFLAGS, as it's actually used by make for default flags