Blender Git Loki

Asset Browser: Avoid appending asset data-block when drop operator will fail

For assets, the copy callback of the drop-box would append the asset
data-block. Check if the operator's poll succeeds before calling the copy
callback.
Otherwise the data-block is "secretly" appended, which the user doesn't expect
and won't notice without checking the file data in the Outliner.

For masks I had to extend the poll function, it didn't check context
sufficiently.