Even more mirror fixes. (#1429)

#1271 Add support for mirrors that do not have avatars in them.  It does this based on if the probe is set to dynamic or not.

#1370 More optimization work to reduce GPU utilization.

#1058 - Removed sim feature flag that was overriding mirrors enabled flags

secondlife/viewer-private#128 - Locally cache the mirror probe

#679 Add additional options to the build floater for mirror probes, including descriptions.