Blender Git Loki

Color management: support of configurable input color space

This adds a user-defined input color space name for image and
movie clip data blocks. Support for this thing for other data
blocks is still a TODO.

Input color space is being used on loading file to convert
loaded image buffer from this space to scene linear space,
later this space is never used in a pipeline.

Color space name was wrapped into own structure like it's
done for display and view settings. This helps keep code
de-duplicated in RNA and it'll help when likely more options
are added into input color space settings (i.e. flag whether
image could be color managed or it's a non-color data).

This implied quite of internal refactoring:

- Made routines around threaded display buffer calculation
more general, so any kind of color transform could be
performed using the same technique.

- Added function to convert given float buffer from given
input color space to output color space. This is a public
function which could be used in such a things as compositor
node.

- Added function to convert ImBuf's content to scene linear
space. Currently used only for images and clips. Should be
added to any image/movie-related data blocks such as
sequencer strips, but that a bit more long-term plan.

- If input color space is set to NONE then no buffer transform
would be performed on image loading. It'll behave in the same
way as using scene linear as input space.