Merge branch 'master' into ui-asset-view-template

Initial AssetList API & use it to load assets for the asset view tempate

Lots of hacks and temporary code here. I wanted a first working version to find
possible pain points before getting into details.

Basic idea is to store each asset library in an `AssetList`, with a globally
accessible storage. Internally the list uses the File Browser's `FileList`
which already does a fair amount of heavy lifting (threaded file & external
asset reading, lazy loading of visible previews, ...). The File Browser could
access the global asset-list storage as well.

The asset view template uses the new asset list to load and display the assets.

Current state: {F9856940}

Open TODOs: https://developer.blender.org/maniphest/query/M6RWFxFSDor3/#R

Build-system: Use C-linkage for internal interface header

That way the header can be included in C++ files.

Icons: Resaved blender_icons.svg in new Inkscape version

Inkscape often does a number of unrelated changes when saving an SVG, not sure
if these are just formatting changes in the source SVG text or if there's more
to it. Either way this makes adding new icons hard since you want to avoid
these unrelated changes. Saving the file in a new Inkscape version every once
in a while should mitigate the problem a bit.
Regenerating the icons (e.g. through `make icons`) shows no difference in the
output.

Note that I also ran "Clean Up Document" in Inkscape, but that doesn't seem to
have changed anything/much.

Icons: Update blender_icons.svg to have a document background

When opening the SVG you wouldn't even see the icons, since they are all white
now. You'd always have to set up a document background color and make sure to
undo that again before saving the file, so it's not written into it.
Just make everyone's life easier by saving the file with a proper background
(gray with checkerboard).

Icons: Add icon for the upcoming spreadsheet editor

The editor is being worked on by the geometry nodes team.
Icon created by Andrzej Ambroz (jendrzych).

Addresses https://developer.blender.org/T85925.

Fix incorrect icon .dat files after recent commit

Using `make icons` for 420f538fadfd changed some icons that it shouldn't have
touched on my system. Not sure what caused this, maybe a bug in the slightly
outdated Inkscape version there (1.0.1). Here on macOS the version is only
reported as 1.0 so I'm not sure what the situation is. Either way, this fixes
the icons.

Fix line width broken for consecutive line draw calls

D9054 did multiple consecutive `immBegin()`/`immEnd()` draw calls to draw
multiple lines at varying thickness. This would only work for the first line,
then they'd all get a 1px thickness (at least on macOS).

Issue was that `wide_line_workaround_end()` called `immBindShader()` directly
to restore the old shader (which the workaround overrides). However this
doesn't set `imm->builtin_shader_bound` which has to be done for the workaround
to work on the next `immBegin()` call. Instead `immBindBuiltinProgram()` can be
called.

Differential Revision: https://developer.blender.org/D10520

Reviewed by: Clément Foucault

Add initial asset-view template, with dummy contents (palette colors)

Idea is to be able to display a list of assets in a layout, by giving it
some custom filtering settings (not done yet). This would be just a
uiList managed by Blender, displaying previews similar to the Asset
Browser.
I decided to use uiLists for this because it already deals with stuff
like writing its UI data to .blends (e.g. filtering and sort settings),
filtering and scrolling. But I think we'll have to do a number of
general improvements to them. Here I just added a new (C-only for now)
"flexible grid" list display type, which uses bigger previews and
behaves much better than the currend "grid" one.

There's a bit of hacking going on here and things will probably change.
For now I'm just interested in getting the basics done to see if the
direction is good.

Fix memory leak when loading previous preferences from splash screen

When opening a Blender version for which there are no preferences, the splash
shows a button like "Load 2.92 Settings". Using this could cause a memory leak
of the storage for recently opened files.