Fix bug in CCLabelBMFont::m_bIsOpacityModifyRGB when changing font file

When the font a CCLabelBMFont uses is changed via setFntFile the m_bIsOpacityModifyRGB property is not updated to reflect whether the new texture uses premultiplied alpha. This can cause incorrect rendering if the label was created without a font file originally, such as through a call to CCLabelBMFont::create().

The fix updates the property through its mutator function to ensure any glyph sprites already created are updated.