gdImageScaleTwoPass memory leak fix

Fixing memory leak in gdImageScaleTwoPass, as reported by @cmb69 and
confirmed by @vapier.  This bug actually bit me in production and I'm
very thankful that it was reported with an easy fix.

Fixes #173.
BGD_DECLARE(gdInterpolationMethod) gdImageGetInterpolationMethod(gdImagePtr im)
{
    return im->interpolation_id;
}

BGD_DECLARE(gdInterpolationMethod) gdImageGetInterpolationMethod(gdImagePtr im)
{
    return im->interpolation_id;
}
