[Impeller] make drawAtlas always use porterduff or vertices_uber shader (#52348)

The drawAtlas component of #52345 . DrawAtlas did not have the same advanced blend/overlapping issues as draw vertices because of the sorting and generation of the sub-atlas, but given that the vertices uber shader exists we can just use that instead.

Will eventually allow deleting the other drawVertices special case shaders.

Author: jonahwilliams