[TODO] Support nested layer name

[syoyo]

Describe the issue

https://openexr.readthedocs.io/en/latest/ReadingAndWritingImageFiles.html#layers

Layer can be nested, but current TinyEXR's ChannelsInLayer API does not support nested layers.

To Reproduce

EXR with nested layer name. e.g. "diffuse.left.R", "diffuse.right.G"

Expected behavior

Support nested layer in some way.

Environment

N/A