oiio

Minor declaration fix

Add maketx --filter to choose good better filtering for MIPmap generation than bilinear interpolation. In the process, this also implemented ImageBufAlgo::resize, overhauled ImageBuf iterators, refactored the API for Texture2D to make it understand separable filters explicitly, and added Lanczos filters.

iconvert --noclobber does not overwrite if the output file already exists.

Epsilon added in targaoutput.cpp

maketx: --oiio option also enable hash generation