Add support for mirrors that don't render avatars.

[Geenz]

Test plan

1. Create a mirror using the Mirror (Environment) option.

2. Verify your avatar and other avatars and their attachments do not reflect in the mirror.
3. Verify everything besides avatars reflects in the mirror.

Description

Useful for clubs and builds with various polished surfaces (such as a near-mirror finish floor).

[sergiiiproductengine]

Deferred QA.

Verified on the Second Life Release 7.1.8.9103842320 (64bit) on Win10/OSX in the scope of https://github.com/secondlife/iqa/issues/239

Could you please provide test plan?

[Dan-Linden]

Passed QA on 7.1.8.9375512768. Tested on Windows/OSX