Blender Git Loki

Use the same string for DNA enum and CLI argument for EXR image format setting

The `--render-format` CLI option takes `EXR` and `MULTILAYER`, whereas
the DNA image format render setting uses `OPEN_EXR` and
`OPEN_EXR_MULTILAYER`. This commit adds the DNA values to the CLI
argument, so that it is possible to take the selected value from DNA and
pass it as-is to the CLI. This is used in Flamenco, for example.

The `OPEN_EXR` and `OPEN_EXR_MULTILAYER` are now aliases, so both the
existing and the new options keep working.

Reviewers: campbellbarton, brecht

Reviewed By: brecht

Subscribers: fsiddi

Tags: #bf_blender_2.8

Differential Revision: https://developer.blender.org/D4502