Fix compiler warnings about set but unused locals
TypeDesc: add handy is_vec3 and is_vec4 methods
Timer: make a lap_ticks() method
Minor simd.h tweaks
simd - minor tweaks
Add an option to link against static ilmbase and openexr libraries, taking care to link in the proper order so that all necessary symbols are included
GIF input plug: Fix build with giflib>=5.1

From giflib-5.1.0's NEWS:
"A small change to the API: DGifClose() and EGifClose() now take a
pointer-to-int second argument (like the corresponding openers)
where a diagnostic code will be deposited when they return
GIF_ERROR."
Support for movie files, using ffmpeg.
Individual frames look like "subimages" to OIIO.
Exclude too-old ffmpeg installations. Also suppress console warnings.
Fix compile errors on MSVS2013.

Our working theory is that one of the MSVS headers has a #define of IN or
OUT. So we change the names of the types in our template, and people report
that it compiles.

Fixes #974
JPEG support for reporting and controlling chroma-subsampling value
Minor docs fixes
Turn off ffmpeg by default.
This is because it's opening other files! Ick. Need to make it exclude
non-movie files.
OpenEXR fix writing of subimage name.
(Got attribute name translation backwards.)
Python - make all IBA functions release the GIL for their expensive part.

The notorious GIL (global interpreter lock) prevents other Python threads
from executing while our functions are holding the lock.

We use a ScopedGilRelease found in many places on the web. Its purpose is
to release the GIL before expensive work, then properly re-acquire it again
when it exists scope. This allows other Python threads to advance while we
are doing the meat of our work.

In many case, this meant writing a wrapper (to scope the GIL business)
where before we did not need a wrapper.