Added test for markers compared with points.

Both are rendering the same SVG marker, at the same size. The
output should (and does, for Cairo & Grid renderers) look like
identical rows of spots. But the AGG one does not.

This is an attempt to create a test case for issue #2117.
Merge pull request #2118 from MapQuest/mla-expr-v2-svg-premultiply

Added test for markers compared with points.