Fix bug with 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 (e.g. CCLabelBMFont::create() ).

The fix updates the property through its mutator function to ensure any glyph sprites already created are updated.