Fix #30858: UI losing buttons at some random moment after using Blender for a

Fix #30858: UI losing buttons at some random moment after using Blender for a
while. This may not fix all cases but should at least solve the issue when
rendering with cycles.

The cause was a race condition on C->data.recursion, with multiple threads
accessing context at the same time. Cycles itself does not access context
from the render thread, but the bpy api would do a context update for any
callback in case e.g. a new file got loaded. Disabled that now in non-main
threads.

The ideal solution would be to not allow any context access at all from threads
but that's not so simple to implement, especially not this close to release.