svga: Follow buffer usage semantics properly.

svga: Follow buffer usage semantics properly.

It's necessary to download buffers from the host always, except if the
buffer is undefined, because:
- just PIPE_BUFFER_USAGE_CPU_WRITE doesn't guarantee all data is written
  -- old contents may still pierce through
- PIPE_BUFFER_USAGE_DISCARD refers to a range, not the whole buffer, so
  unless we track which parts have been modified and not we still need
  to download the data.