when doing get_node_as_image(), that may spawn a new buffer writer that
writes into the samme buffer when rendering an offscreen with patterns.
So as a more or less hacky workaround, we now abort the current buffer
write and restart it once we've created the image.
If creation fails, create an offscreen image instead and draw that as a
texture.
Because offscreens basically always succeed, we can pretty much assume
success everywhere - apart from pattern creation functions that also
create images, because they can run out of shader space.
