This adds support of movie textures for Cycles rendering.
Uses the same builtin images routines as packed/generated images,
but with some extra non-rna hookups from blender_session side.
Basically, it's not so clear how to give access to video frames
via C++ RNA -- it'll require exposing ImBuf to API, doing some
threading locks and so. Ended up adding two more functions which
are actually bad level call, but don't consider it's so much bad
-- we have few bad calls already, which are actually related.
Changed a bit how builtin images names are passing to image
manager. Now it's not just an ID datablock name, but also a frame
number concatenated via '@' character, which makes itpossible to
easily know frame number to be used for movie images, without
adding extra descriptors to image manager.
Decoding of builtin name is a bit slower now, but it should be
still nothing in comparison with rendering complexity.
Also exposed image user's frame_current to python API, which
is needed to get absolute frame number of movie from node's
image user.
P.S. Generated/packed images are also using bad level call but
only does it to make things more clear here. Either all images
are using C++ RNA here or no images does. That's the most clear
for now.
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