falsecolor: Add option to load LUT

[norihiro]

Description

Implement a LUT as suggested in #86.

How Has This Been Tested?

Tested both the source and the filter on Linux.

General checklist

• [x] The commit is reviewed by yourself.
• [x] The code is tested.
• [x] Document is up to date or not necessary to be changed.
• [x] The commit is compatible with repository's license.