Fix IWord::colortoString(): Round instead of truncate

[th-we]

Truncating (by simply casting) may result in inaccurate color values.

[th-we]

@kermitt2 Any chance that this can get merged? Our use case is to analyze PDFs that use color coding, but the original RGB values that were used when creating the PDFs will come out altered when simply casting the values to integers instead of rounding.