no render when maker-width = 0 or opacity = 0

there was no way to avoid rendering in some conditions, with this patch the points are not rendered when marker-width or opacity is 0. Thin lines are also not rendered, this avoid to spend extra time rendering them when there are really thin and avoid a safari rendering bug (think lines are rendered wider than they should)
Merge pull request #49 from CartoDB/avoid_render

no render when maker-width = 0 or opacity = 0