Proposed new encoding: bz2tile

[Matt Campbell]

Hello James:

I have implemented the encoding, and so far, it seems to work well, 
though I'll admit I'm testing on fast CPU's (at least 2 GHz).  Please 
allocate an encoding number for this encoding.

Also, a clarification on this encoding:  Unlike ZRLE, a bz2tile 
rectangle does not start with a length field on the wire.  Instead, each 
bz2tile rectangle is represented on the wire by a complete bzip2 stream. 
  This works because libbz2 can identify the end of a bzip2 stream and 
indicate to the caller how many bytes of input were left unused.  It 
does mean that a bz2tile rectangle requires at least 37 bytes, but the 
server could avoid excess overhead by using hextile for rectangles under 
a certain size.

Matt