Need to support vertex_converters in the same way as AGG, asap.
Waiting to move on this until #1281 is mostly done.
fix marker clamping and sync with agg renderer- refs #1309
sync line rendering between grid and agg - refs #1309
sync grid renderer with agg for text/shields/point - refs #1309
sync grid renderer with agg for point_symbolizer - refs #1309
fix render_marker for grid renderer and other minor syncing - refs #1309
sync agg/grid line symbolizer rendering - refs #1309
sync markers rendering between agg and grid - closes #1309
fix grid/markers tests after a57996e - refs #1309
sync agg with grid renderer for polygon patterns - refs #1309