Replace navigator.xr.supportsSession with navigator.xr.isSessionSupported to follow the latest WebXR API spec

rename multiview.attachRenderTarget() to attachCamera()
use isWebGLMultiviewRenderTarget instead of instanceof

Fix the case where there are two objects with the same material in the scene and one has receiveShadow=true and the other has receiveShadow=false.

This also prevents possible extra shader compiles when the same material is reused with receiveShadow enabled or disabled in the same scene.