Fixed loading of the EXR files with zero alpha and non-zero colors

EXR works with premultiplied colors, so it suports having zero alpha and
non-zero colors. Krita doesn't support premultiplied colors, so we just
set alpha to HALF_EPSILON on loading and reset it back to zero on saving.

This introduces subtle round-trip bugs, but they happen for alpha < 0.01
values only (which is 1%).

TODO:
Now we also need to reset default projection color of the image to black
to load the EXR properly

CCBUG:301443