add Write Texture Format

[TodicaIonut]

Texture Format:
Plain Texture for MIP-mapped OpenEXR files.
CubeFace Environment for environment maps.
Latlong Environment for environment maps.
Shadow for One level.

@devernay @YakoYakoYokuYoku @rodlie @acolwell you please review before I merge?

I only basic testing

[YakoYakoYokuYoku]

This LGTM, although before merging please wait for the approval of another reviewer too.

[TodicaIonut]

OpenEXR TextureFormat here. cc @devernay

"Plain Texture" for MIP-mapped OpenEXR files, "CubeFace Environment" or "Latlong Environment" for OpenEXR environment maps. Non-environment non-MIP-mapped OpenEXR files will not set this attribute.

[devernay]

Hi, why not use all the textureformat values suggested here, and why not use those descriptions from the oiio doc as help strings?