#2 v1.0 example code incorrect

[Ed Hanway]

Instances of example plug-in code deviating from spec:

1. Rectangle.cpp isIdentity()

isIdentity() ignores kOfxImageEffectPropRenderScale
passed with inArgs, can return incorrect result for
render scale != 1.0
To fix, copy the same logic from the render() function.

1. Rectangle.cpp describe()

wrong index used for kOfxImageEffectContextGenerator;
gets overwritten by kOfxImageEffectContextGeneral and
causes problems for hosts that expect multi-dimensional
properties to be initialized in order

1. Custom.cpp customParamInterpFunction()

line 150: result set as property of inArgs; should be
outArgs.