Fix bad transpose logic in aggregate

[thomasp85]

This PR fixes a bug I uncovered while working on my talk.

The following query would fail but is now fixed:

SELECT * FROM ggsql:penguins 
VISUALIZE bill_dep AS x, bill_len AS y 
DRAW point 
  MAPPING species AS fill 
DRAW range 
  MAPPING bill_dep AS xmin, bill_dep AS xmax 
  SETTING aggregate => ('y:mean', 'xmin:min', 'xmax:max') 
  PARTITION BY species

The issue is that aggregate does not correctly pick up the layer transposition and fails to resolve the position aesthetics

[teunbrand]

One comment about duplicate logic, one more conceptual.
Nothing too severe to stop a merge.