This patch fix the existing word API for mist and global ambient lighting.
Add deprecated message to disableMist()
Add setUseMist(enable).
Reviewers: dfelinto, campbellbarton, moguri
Reviewed By: moguri
Subscribers: solarlune, jta, brecht
Projects: #bf_blender:_next
Differential Revision: https://developer.blender.org/D148