Add RMSE based automatic alphamasking for non-attached objects

indra/llrender/llgltexture.cpp

indra/llrender/llgltexture.h

indra/llrender/llimagegl.cpp

indra/llrender/llimagegl.h

indra/newview/app_settings/settings_alchemy.xml

indra/newview/lldrawpoolavatar.cpp

indra/newview/llface.cpp

indra/newview/llselectmgr.h

indra/newview/llviewercontrol.cpp

indra/newview/llviewertexture.cpp

indra/newview/llviewerwindow.cpp

indra/newview/skins/default/xui/en/menu_viewer.xml