renderer: do not use caches in wrong context

With the exception of calling vrend_renderer_transfer_write_iov through vrend_transfer_inline_write, we never come down these code-paths with the correct context active. Trying to do any caching based on this will only lead to incorrect behavior.

There's little point in trying to fixup the caching in the ctx0 for this call-site, as there's no other code-path using ctx0 that tries to play ball with the cache.

So let's get rid ot the context-argument when possible, and pass a NULL-pointer when it can come either from ctx0 or a rendering-context.