OpenEXR loading performance optimizations.

Several improvements:

• Multithreaded reading of files. More cores drastically improve performance with larger files.
• Loaded Exrs will now have default transparent background color.
• Fixed crash when saving exr when flattening.
• Properly loads exrs with different display and data windows.

Differential Revision: https://phabricator.kde.org/D12972

(cherry picked from commit c94a09030c8662c2da9c88533f827af04c77876d)