Error printing for histograms
histogram tests
Fix many compiler warnings and beef up I/O error checking:
* Many cases where we were ignoring the results of fread or fwrite, so
  now we religiously check the results and set errors appropriately
  (this affected bmp, fits, ico, jpeg2000, sgi, targa, tiff, webp, zfile).
* Some unused variables.
This work was mostly Kevin Brightwell, with a cleanup pass by Larry Gritz.
Fix missing OpenEXR open() error message; clean up error reporting in iconvert.
iv refactoring: Split IvGL class definition into ivgl.h (from imageviewer.h).
Fix compiler warning
Compiler warning fixes
Avoid generating iv man pages when USE_QT=0
New texture tests
IC/TS attribute 'substitute_image' lets you force all texture refs to a single image.
oiiotool --fill
New testtex options: --aniso, --stblur
safe_sqrtf clamps to avoid NaN
Anisotropic texturing overhaul and fixes.
(a) Proper computation of the major axis on which to place the samples;
    previously, it was assuming the major axis was the same as the larger
    derivative.  That's true if the derivs are orthogonal in texture st
    space, but not for arbitrary angles.  Solving this fixes a number of
    artifacts, that tended to be only visible with added "blur", but were
    pretty ugly with blur discontinuities.
(b) Adjusted the strategy we were using for "conservative" dealing with
    extreme anisotropy to get a better visual result.
(c) Eliminate a 2x error where we were not distributing the samples far
    enough along the line.
(d) Apply Gaussian weights to the samples.
Update testsuite reference images
Merge pull request #417 from lgritz/lg-texblur

Anisotropic texture blur fixes
Guard against numeric problems with texture filter ellipse computations.
Found cases where our shortcut in nsamples was leading to aliasing. So
switch to the theoretically correct nsamples = 2*aspect-1, eat the minor
extra cost to avoid aliasing of patterns in anisotropic regions.
Filter ellipse refinement: simplify math with new (equivalent) formula
confirmed by computer algebra program.
Merge pull request #422 from lgritz/lg-texblur

Texture blur fixes