Fix OpenEXR brittleness with malformed metadata.
Our particular case was a string called "tiles" -- triggers an exception
inside libIlmImf.
Fix errors in mitchell and gaussian default width. Fixes #835.
Mitchell should be 4, was incorrectly 3. Gaussian should be 3, was 2.
Note that there has always been "sharp-gaussian", which is 2 by design.
The fact that "gaussian" was also 2 is clearly a typo, there's no reason
for us to have intentionally made "gaussian" and "sharp-gaussian" identical.