CoreProfile multisampling is not working on OSX

when a coreprofile is used on osx, reading the z value from the depthbuffer for picking, the glReadPixels call from glViewer::handleNormalNavigation QtBaseViewer.cc:1924 causes a glerror.

as a temporary workaround multisampling was disabled by setting the sample count to 0 in coreWidget.cc:220 this value should be a setting anyways, so when it is properly implemented as a setting also fix the DepthBuffer reading. a potential solution would be something similar to the snapshot function, but rendering everythign twice for mouse movement handling seems quite bad for performance.