Fix RTF shape border color mapping

[vixco]

Summary

• Include m_nBorderRightColor when copying line color to picture border colors.
• Write o:borderbottomcolor from m_nBorderBottomColor.
• Write o:borderrightcolor from m_nBorderRightColor.

Why

Two RTF shape border color mappings were inconsistent:

• The line-color fallback assigned m_nBorderTopColor twice and never assigned m_nBorderRightColor.
• Shape property rendering emitted bottom border color from the right field and right border color from the bottom field.

Both cases can make a shape or picture border use the wrong side's color when round-tripping RTF.

Validation

• Ran git diff --check.
• Checked the border color field definitions.
• Checked the RTF shape property reader and VML-style shape rendering for the same border color fields.