re-ordered the methods
I forgot to initialize vradMeta->missing, so vol2bird used whatever value was in that memory location, which usually worked (as in did something, but not the right thing), but sometimes resulted in -nan values, ultimately manifested by small differences between the unit test results and the vol2bird result.
added a fixme for issue #44
added a missingValue value to the call to calcTexture...now passes all unit tests successfully