Rear view zoom level

new zoom icons
save and restore state from settings
display zoomed out rear view when selected
Merge branch 'master' of https://github.com/worklist/hifi into 19447

Conflicts:
	interface/src/ui/RearMirrorTools.cpp
fix packetData memory leak in AvatarMixer
add debug to catch AvatarMixer ghosting
remove the ground plane
Starfield fix and Milky Way

Fixed bug where more stars were generated at the poles.

Added 'Milky Way' band in sky, customisable with MILKY_WAY_WIDTH,
MILKY_WAY_INCLINATION and MILKY_WAY_RATIO consts in Generator.cpp.
add debug for sleep amount to AvatarMixer
Merge pull request #1143 from birarda/assignment

add node kill debugging, remove ground plane
made sending environments off by default
Merge pull request #1144 from ZappoMan/stephens_requests

made sending environments off by default
have silent node removal flag reset on join
reference global with colon
first cut at user adjustable LOD
first cut at user adjustable LOD
Merge pull request #1145 from birarda/assignment

fix silent node removal thread join bug
added --getOctalCode to voxel edit
Merge pull request #1146 from ZappoMan/stephens_requests

added --getOctalCode to voxel edit so you can easily make jurisdictions from arbitrary voxels
Normal map support almost there.
more work on LOD slider
make server resend scene when LOD changes