Handle visibility:hidden and visibility:visible

[porglezomp]

This tracks visibility:hidden and visibility:visible in addition to display:none. visibility is able to be reset, unlike display, so we track it separately, and make the path visible as long as both are set.

To avoid adding another field to the struct, I put it in with bit-packing in the visible field, but I could swap it to using a different field if you think simplicity would be worth the extra bytes.

[SergeySlice]

It will be good to handle also group visibility

[porglezomp]

It's been a while since I've looked at this code, but I believe I recall it handles group visibility using the same attribute stack as everything else. That's what I'm describing with "visibility is able to be reset."