exr: simplify decompression path

Signed-off-by: Paul B Mahol <onemda@gmail.com>
Include fix for building ismindex under MinGW

Include cmdutils.h so we can build under windows. 

Based on fix for https://ffmpeg.org/trac/ffmpeg/ticket/256
Merge remote-tracking branch 'fredjean/master'

* fredjean/master:
  Include fix for building ismindex under MinGW

Merged-by: Michael Niedermayer <michaelni@gmx.at>