Fix saving/loading of exr images with small alpha values

There are several issues possible:

1) The image has zero alpha + non-zero color pixels. We artificially raise
the alpha value to allow premultiplication.

2) The image has tiny non-zero alpha + non-zero color pixels, but division
is possible without overflow of color channels. Then we must not change
anything.

3) The image has tiny non-zero alpha + non-zero color pixels, but division
is not possible due to a half overflow. In this case we must also raise
alpha a bit (as in case 1) ) to allow correct rendering of the image.