Initialize MergingDrawBatch clip with viewport bounds

This allows merged, clipped operations to behave correctly within a
savelayer, even if the base viewport has a large offset.

Additionally, disregard opaqueness when within a
complexclip/savelayer, as the coverage can't be trusted.

Change-Id: Ic908b82a4bb410bc7fac1b4295f4874ed166efc5