Properly clear gutter markers in Sandcastle

I was using `clearGutter` incorrectly, which resulted in the problem.
In order to match the old behavior, we now declare 4 gutters and clear them
out as needed.
Merge pull request #2228 from AnalyticalGraphicsInc/sandcastle-gutters

Properly clear gutter markers in Sandcastle
Merge remote-tracking branch 'agi/master' into quantizedMeshWaterMask