fixed default implementations of renderer::Source::evaluate() variants returning color+alpha.
fixed default implementations of renderer::Source::evaluate() variants returning color+alpha.

pjessesco/appleseed,aiivashchenko/appleseed,gospodnetic/appleseed,Aakash1312/appleseed,aytekaman/appleseed,aiivashchenko/appleseed,luisbarrancos/appleseed,luisbarrancos/appleseed,dictoon/appleseed,Vertexwahn/appleseed,est77/appleseed,aytekaman/appleseed,est77/appleseed,gospodnetic/appleseed,glebmish/appleseed,Aakash1312/appleseed,dictoon/appleseed,gospodnetic/appleseed,glebmish/appleseed,est77/appleseed,docwhite/appleseed,glebmish/appleseed,est77/appleseed,Vertexwahn/appleseed,gospodnetic/appleseed,appleseedhq/appleseed,appleseedhq/appleseed,docwhite/appleseed,Biart95/appleseed,dictoon/appleseed,glebmish/appleseed,docwhite/appleseed,Vertexwahn/appleseed,luisbarrancos/appleseed,gospodnetic/appleseed,Biart95/appleseed,pjessesco/appleseed,est77/appleseed,aiivashchenko/appleseed,Aakash1312/appleseed,dictoon/appleseed,glebmish/appleseed,dictoon/appleseed,aiivashchenko/appleseed,Vertexwahn/appleseed,aytekaman/appleseed,luisbarrancos/appleseed,Biart95/appleseed,docwhite/appleseed,aytekaman/appleseed,docwhite/appleseed,Vertexwahn/appleseed,Biart95/appleseed,Biart95/appleseed,pjessesco/appleseed,appleseedhq/appleseed,Aakash1312/appleseed,Aakash1312/appleseed,aytekaman/appleseed,luisbarrancos/appleseed,appleseedhq/appleseed,appleseedhq/appleseed,pjessesco/appleseed,pjessesco/appleseed,aiivashchenko/appleseed