Bug 21987: Do not generate true color thumbnails if not needed

Bug 21987: Do not generate true color thumbnails if not needed

If the original image is not a true color image we should not generate a
true color thumbnail.

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
(cherry picked from commit 5c41d584cb654efbfd8c854bac0d30d87a054ab5)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 8928ba93250633eb82199e06c2198487b955d734)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>