Only call lineTo if we are in a new pixel for linestring

Rendering performance degrades as the context lineWidth increases when calling lineTo many times for the same pixel coordinate.  This appears to be a widespread performance issue and is very clearly improved by only calling lineTo when in a new pixel coordinate.
Avoid calling lineTo on the same pixel for polygon stroke
Merge pull request #1132 from tschaub/wide-stroke

Only call lineTo if we are in a new pixel.