Leica LIF: do not set Channel.Color for RGB images

The stored color will usually (always?) be 0xffffffff, which causes the
displayed colors to be inaccurate (since the 'R' component will be shown
as white).

Fixes #12214.
Merge pull request #1193 from melissalinkert/12214

Leica LIF: do not set Channel.Color for RGB images