High bit-depth grayscale images: respect the user's tone-mapping preference

Grayscale images use a different pipeline, and it was set to always
tone-map incoming HDR images.  Now they will be handled according to the
main tone-mapping preference in the Options dialog.
Custom radio button: mirror optimizations from check box

See c722cd288af104e108e184ad3cfccace12a0f20a for details