fix a bug where LabelTTF::setFontFillColor() won't work as expected

In CCDevice.mm, Device::getTextureDataForText() function, info.strokeColorX and info.tintColorX are float type (0-1), which are assigned the value of GLubyte (0-255). The values need to be converted.
LabelTTF::setFontFillColor() will work properly once the fixes are made.