Restore OpenEXR support to ocioperf

The ocioperf command-line tool is used to do performance tuning. Originally it used OpenImageIO to load an image file to be processed for the test. However, if OpenImageIO was not present, ocioperf would not get built. Therefore, ocioperf was modified to generate an image buffer internally, so there is no dependence and the tool always gets built. However, more recently, OCIO now is able to download and build OpenEXR automatically.

This issue is to restore the ability for ocioperf to use custom image files (in .exr format) by leveraging the fact that OpenEXR will always be available to build the tools.