Use Ray Cast only on Apple Mac OS X
Use Ray Cast only on Apple Mac OS X

After some investigation it is clear that the texture mapping code
is not able to render volumes at the typical sizes required by
tomviz, as Apple cannot do GPU accelerated volume rendering it tends
to fall back to Ray Cast and Texture Mapping, resulting in crashes.
For now force Apple machines to use Ray Cast only.

cjh1/tomviz,Hovden/tomviz,OpenChemistry/tomviz,thewtex/tomviz,thewtex/tomviz,mathturtle/tomviz,mathturtle/tomviz,yijiang1/tomviz,cryos/tomviz,thewtex/tomviz,yijiang1/tomviz,OpenChemistry/tomviz,cryos/tomviz,mathturtle/tomviz,cjh1/tomviz,cjh1/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,Hovden/tomviz,cryos/tomviz