Auto-detects text_anchor, text_justify, text_offset_right, text_offset_down
columns in the source sf for per-feature label placement.
Arguments
- m
map or map_proxy
- labels
list of label specs, each with: source (sf), text_field, and
optionally id, text_color, text_size, text_font, text_halo_color,
text_halo_width, text_halo_blur, text_line_height, text_allow_overlap,
filter, text_anchor, text_justify, text_offset