Create a spec ready for compiling to video

Usage

er_spec(
  header,
  clips,
  audio_tracks,
  theme = "er_theme_default",
  as = "object",
  ...
)

header: er_header: as returned by er_header(). Alternatively, do not specify the header argument but pass named header parameters separately
clips: list: a list of er_clip objects as returned by er_clip()
audio_tracks: list: a list of er_audio_track objects, as returned by er_audio_track()
theme: string: name of a theme function to apply, or NULL for no theme. The theme will set defaults, but will not override any defaults specified in the header
as: string: either "object" (return an object of class er_spec, suitable to pass to er_to_json(), or "json" (the JSON string as obtained from er_to_json())
...: : as for er_header

A list of class er_spec or a JSON string