It looks to me like we will end up premultiplying twice in agg_renderer::setup if a background image (with alpha) is used with a background color with alpha after 8c98d8a. Actually could even happen three times as markers are premultiplied during cache population after 3272707.
-avoid potentially premultiplying colors more than once in agg_render…
…er::setup - closes #1515
update tests after #1515 - many are not ideal due to #1519