HDRI support

[RobertBeckebans]

It's time to bring in support for skies that you can drag n drop into Blender and Substance Painter.
This should be .exr format and converted to R11G11B10 format as long we don't have support for BC6 compression.

[Calinou]

and converted to R11G11B10 format

Don't GPUs support the RGBE9995 format with uncompressed data? This should provide better precision with fewer exposure limits. One of the points of HDRIs is that they can get pretty bright after all, so R11G11B10 may be too limiting even if you use a 2× exposure multiplier. Higher multipliers may result in visible banding.