erroneous garmin gpi warning when writing with a 24 or 32 bits per pixel bitmap

[tsteven4]

When using the bitmap option with a bitmap that don't use a color table (e.g. 24 or 32 bits per pixel) an inappropriate warning may be output such as
garmin_gpi: Code error in load_bitmap_from_file, expected output size 1060, actual output -28.

The warning is only valid for bitmaps that use color tables (e.g. 8 bits per pixel).

[tsteven4]

fixed in #1243