No one should every try to write to this buffer. Enforce that by
throwing an exception if any one tries to get a writeable pointer
to the data.

If an area recently changed then we can guess that it will most likely
change again very soon. In such a case it is meaningless to send a
lossless refresh as it will directly be overwritten. Keep track of
such areas and avoid refreshing them until we no longer see any
changes to them.

We don't want to waste bandwidth on the lossless refresh if we might
need that bandwidth for a normal update. Try to estimate how much
data we can safely send without interfering.

Resend pixel perfect copies of areas that were previously sent
using a lossy encoder. This is done when there is no normal update
to send, and no congestion.

Clang doesn't like when struct and class are used to refer
to the same thing interchangeably.

Fixes all instances of:

In file included from /home/shade/dev/tigervnc/common/rfb/CConnection.cxx:29:
In file included from /home/shade/dev/tigervnc/common/rfb/CConnection.h:28:
/home/shade/dev/tigervnc/common/rfb/DecodeManager.h:35:3: error: class 'Exception' was previously declared as a struct [-Werror,-Wmismatched-tags]
  class Exception;
  ^
/home/shade/dev/tigervnc/common/rdr/Exception.h:32:10: note: previous use is here
  struct Exception {
         ^
/home/shade/dev/tigervnc/common/rfb/DecodeManager.h:35:3: note: did you mean struct here?
  class Exception;
  ^~~~~
  struct
/home/shade/dev/tigervnc/common/rfb/DecodeManager.h:43:3: error: class 'Rect' was previously declared as a struct [-Werror,-Wmismatched-tags]
  class Rect;
  ^
/home/shade/dev/tigervnc/common/rfb/Rect.h:68:10: note: previous use is here
  struct Rect {
         ^
/home/shade/dev/tigervnc/common/rfb/DecodeManager.h:43:3: note: did you mean struct here?
  class Rect;
  ^~~~~
  struct

The previous FIXME was incorrect. The old code did in fact continue
looking over the entire area. This commit restores that old behaviour.

It knows more details so it can track things better.

This allows us to apply a lot more server logic
independently of which encoder is in use.

Most of this class are things moved over from the
Tight encoder.