Fix T93649: Blender freezes when saving with active VR session

Dead-lock when VR viewport drawing and depsgraph updates would fight for
the draw-manager GL lock. This didn't usually cause issues because the
depsgraph would be evaluated at this point already, except in rare
exceptions like after file writing.

Fix this by ensuring the XR surface gets its depsgraph updated after
handling notifiers, which is where regular windows also do the depsgraph
updating.

Fix possible use-after-free on error handling during VR view drawing

Whenever an exception happens in VR session code, we cancel the entire
session. Alongside that, we removed the "surface" item used to draw into
an offscreen context. But this may still be stored as active surface,
leading to a use-after-free when deactivating this active surface, for
example.

Fix null-pointer dereference on error handling during VR view drawing

Fix possible use-after-free on error handling during VR view drawing

Whenever an exception happens in VR session code, we cancel the entire
session. Alongside that, we removed the "surface" item used to draw into
an offscreen context. This would mess up the iterator of the surface
draw loop.
Similar to 7afd84df409a.

Fix possible use-after-free on error handling during VR view drawing

Whenever an exception happens in VR session code, we cancel the entire
session. Alongside that, we removed the "surface" item used to draw into
an offscreen context. This would mess up the iterator of the surface
draw loop.
Similar to 7afd84df409a.

Fix null-pointer dereference on error handling during VR view drawing

Fix possible use-after-free on error handling during VR view drawing

Whenever an exception happens in VR session code, we cancel the entire
session. Alongside that, we removed the "surface" item used to draw into
an offscreen context. But this may still be stored as active surface,
leading to a use-after-free when deactivating this active surface, for
example.

Fix T93649: Blender freezes when saving with active VR session

Dead-lock when VR viewport drawing and depsgraph updates would fight for
the draw-manager GL lock. This didn't usually cause issues because the
depsgraph would be evaluated at this point already, except in rare
exceptions like after file writing.

Fix this by ensuring the XR surface gets its depsgraph updated after
handling notifiers, which is where regular windows also do the depsgraph
updating.

Fix T93691: Crash when loading custom thumbnail in custom library

This was an issue with the mixed list of external assets and assets from
the current file. When closing the File Browser to select the custom
preview image, the assets from the current file would be cleared for
reread, to make sure we display up-to-date file data. That is because
the workspace of the temporary File Browser was deleted, causing a
change in the file data (main data-base). The reread would happen in a
background thread, meaning it might not finish before the custom preview
operator runs and queries the active asset. So the preview operator
would get the wrong active asset from context.

Two fixes were needed:
* Make sure current file data is reread before the operator runs, by
doing this partial rereading on the main thread.
* Ensure the asset list (in fact file list) order stays consistent over
rereads. If multiple assets with the same name were shown, the
operator might also have gotten the wrong asset, also leading to a
crash.

Additionally the file operation handler should probably poll before
executing, to fail gracefully at least (not crash).

Cleanup/Documentation: Add/move comments for asset files

Adds some basic high-level explanations for editor/UI level asset APIs.
Also moves one such comment from the source file to the header file,
so it's in the same file as other API comments.