This is a critical fix that should also be backported to 2.83.1
Fairly stupid bug in fact, code detecting changes across undo steps was
assuming that each BHEAD (a block of data in blendfiles) would not be larger
than one memory chunk... Which is the case in alsmost every situation,
besides some super-heavy geometries, and other similar things (images
would also be affected e.g.).
This patch implements the list panel system D7490 for modifiers.
It also moves modifier drawing to a callback in ModifierTypeInfo
in line with the extensible architecture refactoring goal T75724.
This adds a PanelRegister callback and utilities for registering
panels and subpanels. It also adds the callbacks for expansion saving
and drag and drop reordering described in D7490.
These utilities, callbacks, and other common UI elements shared
between modifiers live in MOD_ui_common.c.
Because modifier buttons are now in panels, we can make use of
subpanels for organization. The UI layouts also use the single
column layout style consistently used elsewhere in Blender.
Additionally, the mode-setting buttons are aligned and ordered
consistently with the outliner.
However, the large number of UI changes in this patch may mean
that additional polishing is required in master.
Thanks to William Reynish (@billreynish) who did a fair amount of the
layout work and to Julian Eisel (@Severin) for consistent help.
Differential Revision: https://developer.blender.org/D7498
Optimizations in rBcda15408582e8de5b405 do not guarantee anymore that
consecutive memchunks in two consecutive undo steps are actually about
the same data (and hence can share the same buffer when unchanged).
This buffer sharing can now happen without any particular order, so we
need to change the process when 'merging' two undo memfiles together.
Note that existing code was not logically correct either, even with
previous undo storage code, since it would blindly transfer ownership of
the buffer to the second memchunk, without checking whether the first
one was actually the owner of it or not (a same buffer can be shared by
matching memchunks in many consecutive memfiles/undo steps).
