helpers for image rendering

[mdsumner]

we want something like

im <- whatarelief::imagery(source = whatarelief::gebco21())

but, to have that automatically scale the data to a palette (for annotation_gdal - or, should annotation_gdal just do this ??)

thinking functions

raster_data() ## to get data like elevation() does just a better name

image_data() ## to always return colours for any source, needs to handle 3,4 bands and detect if it's already colour-model-ready

[mdsumner]

whoa, maybe do with `vrt://...?scale' args ..

[mdsumner]

first we need vapour to not apply scaling