buffer_base: Add support for cached CPU writes

Some games usually write memory pages currently used by the GPU, causing
rendering issues (e.g. flashing geometry and shadows on Link's
Awakening). To workaround this issue, Guest CPU writes are delayed until
the command buffer finishes processing, but the pages are updated
immediately.

The overall behavior is:
- CPU writes are cached until they are flushed, they update the page
  state, but don't change the modification state. Cached writes stop
  pages from being flushed, in case games have meaningful data in it.
- Command processing writes (e.g. push constants) update the page state
  and are marked to the command processor as dirty. They don't remove
  the state of cached writes.