In situations with complicated DiffMaps, there is a significant
performance cost to doing all of the colour setting and individually
filling and stroking each chunk. This commit avoids this cost by
pre-sorting the chunk list into blocks of the same type, allowing us
to set up all of our paths of a single colour, and stroke/fill in one
Cairo call.

In theory, this may cause visual changes by altering the order in which
we overwrote pixels (i.e., instead of top-to-bottom, we now do
colour-by-colour). In practice, this was never really defined anyway.