virgl: Add tweak to emulate coherent buffer writes
Similar to apitrace emulate the coherent write by using a protected shadow memory to track writes and commit the writes before each draw call. Since without ARB_buffer_storage each time full 4MB buffers are transferred, this reduces the amount of transfers considerably.
The speed gain with Bioshock Infinite is approximately factor 3 (for number see the last commit).
Since only coherent writes are covered this is implemented as a tweak that must be enabled on request.