No fallback on visibility controls (other settings may not either)

[boberfly]

Currently when you specify different visibility flags to objects in Solaris, the viewport will update this state however if you set these controls to "Do Nothing" (essentially removing the attributes from the schema) the viewport doesn't revert back to the Cycles defaults and just sticks to the last thing that it was set to.

However, when re-loading the scene from a fresh Houdini instance or rendering to husk it'll do the right thing, as the defaults are there from a fresh initialisation. I am wondering now if this is just for visibility flags, or that hdcycles might need to have a way to always fallback to defaults for all settings.

[boberfly]

Having a deep-dive if I could solve this one myself, for object/mesh/curve/volume/lights I can set the actual default value by hand from what cycles is set to, but it would be preferred to get the default value from the schema itself.

I also saw all of renderParam.cpp it is just feeding in the previous value as default value, which is wrong as it should be the default value that is set at init time (the default built into cycles values and not just the previous value which will change as the user changes values in a running session).

[boberfly]

Potential fix:
#151