Avoid duplicates when drawing with constant `aes`

[Vindaar]

Fixes duplication of constant geoms.

For example a geom like:

geom_text(aes = aes(x = 1.0, y = 2.0, text = "foo"))

would previously draw the text once for every row in the input DF.

* v0.6.2
- handle geom calls with constant values better. Previously we
  sometimes ended up drawing the same e.g. text multiple times, once
  for each input DF row. We now deduplicate for cases where ~x~ and
  ~y~ are constant.