This makes the (currently single) storage buffer handled by
GskGpuDescriptors.
A side effect is that we now have support for multiple buffers in place.
We just have to use it.
Mixed into this commit is a complete rework of the pattern writer.
Instead of writing straight into the buffer (complete with repeatedly
backtracking when we have to do offscreens), write into a temporary
buffer and copy into the storage buffer on committing.
