Overhaul and API for mirrors

[ctrlaltdavid]

"mirrors/portals currently work by hackily building off of the secondary camera, which is itself a hacky way of rendering a second camera view to a texture. we need to formalize and separate these APIs so the secondary camera can be used for streaming and mirrors can be configured via entity properties (and also, set to be portals) that render using the stencil buffer instead of textures, which will fix the low res mirror problem and give us stereo mirrors" [HifiExperiments]

[stale[bot]]

Hello! Is this still an issue?

[daleglass]

This needs more details about what's wrong with the current API and what changes could be made to it

[stale[bot]]

Hello! Is this still an issue?