Revert "vnc: allow fall back to RAW encoding"

Revert "vnc: allow fall back to RAW encoding"

This reverts commit de3f7de7f4e257ce44cdabb90f5f17ee99624557.

Remove VNC optimization to reencode framebuffer update as raw if it's
smaller than the default encoding.

QEMU's implementation was naive and didn't account for the ZLIB z_stream
mutating with each compression.  Because of the mutation, simply
resetting the output buffer's offset wasn't sufficient to "rewind" the
operation.  The mutated z_stream would generate future zlib blocks which
referred to symbols in past blocks which weren't sent.  This would lead
to artifacting.

Considering that ZRLE is never larger than raw and even though ZLIB can
occasionally be fractionally larger than raw, the overhead of
implementing this optimization correctly isn't worth it.

Signed-off-by: Cameron Esfahani <[email protected]>
Signed-off-by: Gerd Hoffmann <[email protected]>