update_args: Update `xpose_geoms` arguments
Description
Combine the arguments from the user and the `xp_theme`.
Usage
update_args(thm_arg, name, ...)
Arguments
thm_arg
A subset of `xp_theme` used to test defaults.
name
Name of the destination layer for the argument
parsing (e.g. point).
Value
A list of arguments for the layer defined in `name`.