WIP r.external: transfer_colormap added

[landam]

Color maps seems to be not transferred when linking raster data using r.external.

r.in.gdal in=/home/user/geodata/sentinel/sample/L2A_T32UPB_20170706T102021_B04_10m.jp2 out=b4
r.colors.out b4
1 0:0:0
17266 255:255:255
nv 255:255:255
default 255:255:255

vs.

r.external in=/home/user/geodata/sentinel/sample/L2A_T32UPB_20170706T102021_B04_10m.jp2 out=b4l
r.colors.out b4l
nv 255:255:255
default 255:255:255

This causes that a linked raster map is displayed in map canvas as transparent which is very confusing for the user.

New function transfer_colormap() was just taken from r.in.gdal. This solution cannot be accepted since it introduces a code duplication. Unfortunately there is already quite significant code duplication speaking about GDAL-based modules (r.in.gdal, r.external, v.in.ogr, v.external). What would probably feasible to create a new gdalioib which could be used by these modules.

Any opinion?

[metzm]

The changes look good to me.
However, you are right that this causes more code duplication. I am not sure if a new GRASS lib like gdaliolib is needed or if we can put some functionality into the separate libs rasterlib, vectorlib, and projlib (for the nearly identical projection checks in the for modules). This should go into a separate new issue.