This commit changes a lot of how IDs are handled internally, especially the unlinking/freeing
processes. So far, this was very fuzy, to summarize cleanly deleting or replacing a datablock
was pretty much impossible, except for a few special cases.
Also, unlinking was handled by each datatype, in a rather messy and prone-to-errors way (quite
a few ID usages were missed or wrongly handled that way).
One of the main goal of id-remap branch was to cleanup this, and fatorize ID links handling
by using library_query utils to allow generic handling of those, which is now the case
(now, generic ID links handling is only "knwon" from readfile.c and library_query.c).
This commit also adds backends to allow live replacement and deletion of datablocks in Blender
(so-called 'remapping' process, where we replace all usages of a given ID pointer by a new one,
or NULL one in case of unlinking).
This will allow nice new features, like ability to easily reload or relocate libraries, real immediate
deletion of datablocks in blender, replacement of one datablock by another, etc.
Some of those are for next commits.
A word of warning: this commit is highly risky, because it affects potentially a lot in Blender core.
Though it was tested rather deeply, being totally impossible to check all possible ID usage cases,
it's likely there are some remaining issues and bugs in new code... Please report them! ;)
Review task: D2027 (https://developer.blender.org/D2027).
Reviewed by campbellbarton, thanks a bunch.
So now, in the new "other" tex context, you can (depending on active data) have direct access to modifiers', force's or brushes' textures...
I also refactored a bit how texture contexts are handled (once again, we had some quite similar code in both space_buttons and RNA sources). This should also solve some harmless glitches like "no texture context selected in UI" sometimes when you remove data related to current texture (see e.g. after removing the material from default cube, in startup scene).
This usage of two different systems for textures, and the handling of switches between them, has been a bit tricky to get working right, but it is OK now I think. I also had to add a bool flag to buttons space, SB_TEX_USER_LIMITED (use_limited_texture_context in RNA), which indicates "new shading" texture code whether it has to ignore materials, lamps etc. (BI) or not (Cycles).
Btw, pinned textures from modifiers/force/etc. were also broken (showing nothing), now it should work too.
Thanks to Brecht for reviewing.
