Blender Git Loki

Fix T91197: marking assets from Python may crash

When using `asset_mark` function from a Python script and afterwards
updating the preview image, a crash might happen. The preview image is
generated by the `asset_mark` function. This may happen on a background
thread, introducing potential synchronization issues.

This patch fixes this by separating the preview generation
`ID.asset_generate_preview` from the mark as asset `ID.asset_mark`.

Note: this separation of "mark as asset" and "generate preview" also
applies to the `ED_asset_mark_id()` C function; if it is desired to have
previews rendered after marking as asset, a call to
`ED_asset_generate_preview()` is now also required.

Reviewed By: sybren

Maniphest Tasks: T91197

Differential Revision: https://developer.blender.org/D12922