Using geometry nodes with attributes on a curve object and changing the
output is crashing. This is because the `render_mutex` in the curve
drawing cache is cleared after changes in `curves_batch_cache_init` and
set to null. The cache isn't actually needed currently because all draw
updates are single-threaded, but the new `drw_attributes_merge` function
still tries to access it (this seems to be tolerated on Linux platforms
but crashes on Windows).
Make sure the render_mutex is always initialized after (re-)initializing
the cache.
9024ac31be