Blender Git Loki

Fix T66515, T67112, T61607: failure to read EXR files with single, named layer

Like Blender renders without a Z channel. The single layer case assume that channel
names are just R/G/B/A without any layer name prefix, and would not read channels
like "Image.R".

Carefully tested for regressions with the openexr project tests images, so this
should be safe.