node_shader_utils: fix bad setting of use_nodes in readonly case.

Also added access to extension property in texture wrapper.

node_shader_utils: several fixes, improvements and cleanups.

Fix broken behavior in case of texcoords mapping (we do need texcoords
node in all cases, then, even for UV coords...).

Use nodes by default when generating new write-allowed wrapper around a
material.

Do not try to find nodes in existing tree all the time, do it only once,
even for lazy-initialized nodes (through accessors).

Fix ugly spacing in property accessors (since it looks like some people
do not like a single 'block' with both getters, setters and prop
definition, at least use one sep line everywhere (and two sep lines to
separate properties)...

Fix (unreported) wrong 'use_sep' setting for menu items.

This option defines whether or not menu items drawing code must
separate drawstring in two, keeping right part at all cost.

This is used to show shortcuts of operators' enum entries usually.

Previous way to decide that was based on button having an RNA prop
pointer, assuming those without it were not 'data buttons' and hence
needed the shortcut special handling thingy.

That's wrong is many, many cases (especially since search templates
often generate more than one buttons, not all linked to actual RNA
data). So instead now checking whether a button has an optype set or
not, hopefully this will be much more accurate...

Other solution if thsi also fails, is to add new flag to buttons, and
explicitely set it when needed, instead of trying to guesstimate...

Fix T57122: Sample color during texture painting not working.

Stupid +1/-1 fiddling of mat indices (between MPolys and object...).

Initial investigation and patch by @dfelinto, thx.

Fix crash in T56064: Blender crashes on selecting text-object.

The root of the issue remains though, see T56172, this is just a quick
bandaid to stop crashing on it, until we find a proper solution.

Fix wrong RNA handling of some internal pchan pointers.

There were two issues here:
* cutsom_shape_transform was not properly tagged as not-owned pointer.
* custom_shape_transform and bbone_start/_end could be set to a pchan from
another data-block (not from UI, but RNA access code itself allowed it).

Those two issues were specificaly breaking complex rigs in static
override case.

EDIT: since yesterday, bbone_start/_end are mere accessors to edit_bone
data in RNA, so most of fix related to those was ditched (they still needed
to be 'deactivated' from RNA diffing with PROPOVERRIDE_NO_COMPARISON).

Improvements to internals blf_font's `blf_font_width_to_strlen()`/`to_rstrlen()`

`to_strlen` just benefits from using pre-computed kerning table for
ascii chars (gives about 30% speed improvements).

`to_rstrlen` was re-written and heavily simplified, basically using same
logic as `to_strlen`, and `BLI_str_find_prev_char_utf8()` to loop
backward in the string, instead of looping forwards the whole string,
storing each gliph's width in temp array, and looping backward on that
temp array to find final string matching expected width. Gives about 70%
speed improvements!

And both functions can now share their core logic.

UI strings: Fix asserts in 'middle-splitting' fitting string code.

The problem is that string width computing is performed in integers
(pixels), which can generate a rather annoying error (a few pixels)...
Simply work around that for now, by trimming an extra middle char when
needed.

Fix (unreported) memleak with legacy IKsolver.

Looks like new depsgraph may initialize some IK trees, without ever
executing them (which also frees them with legacy IKSolver code)...

Depsgraph/RNA: add warning that all data from object instances iterator are COW data.

It is crucial that scripts do not write, and even more importantly, do
not store any references to those. Otherwise, that?s a rather straight
path to crash.