text -M: Font color is ignored in case -G or/and -W are used

[yvonnefroehlich]

Description of the problem

First reported in GenericMappingTools/pygmt#4590

-M of text allows to set text as paragraphs.
However, the font color is ignored in case a box is added via -G or/and -W.

Full script that generated the error

• input data: text_para.txt

gmt text text_para.txt -R-5/5/-5/5 -JX10c/10c -B -M -F+f10p,1,steelblue -png plot_para

gmt text text_para.txt -R-5/5/-5/5 -JX10c/10c -B -M -F+f10p,1,steelblue -Ggray -png plot_para_g

gmt text text_para.txt -R-5/5/-5/5 -JX10c/10c -B -M -F+f10p,1,steelblue -W1p,gray10 -png plot_para_w

no box	box via -G	box via -W

System information

• Operating system: Windows
• GMT version (gmt --version): gmt 6.6.0, gmt dev