Cleanup: redundant casts
Allow render engine to declare whether it's capable of the texture rendering or not

If render engine has bl_use_texture_preview set to truth blender wouldn't fallback to
the blender internal rendering for previews.
Fix T42106: Box image mapping shows black triangles if they point to a corner and blend is 0

After discussion with cambo here we decided it's better to choose arbitrary side of the box
(in this case it's X-axis) and use image from it. That's better than doing a blackness.

P.S. This is literally a corner case anyway.
Call python3 explicitly from tgz util
Icons: were relicensed to GPL2+ metadata was CC-NC
Partial fix of T42052: Clip editor does not support Continue zoom style