rendering
Make scene background color as default for render-to-texture instead of
current blue color (0, 0, 255).
It is very useful for mirrors setups.
Reviewers: moguri, ben2610, sybren, panzergame, hg1
Reviewed By: panzergame, hg1, moguri
Subscribers: mpan3
Differential Revision: https://developer.blender.org/D1287