Add billboard rotation and aligned axis.
Merge branch 'master' into billboardRotation
Merge branch 'master' into billboardRotation
Merge branch 'master' into billboardRotation
Fix issue with vertical and horizontal origin. Add tests. Update Sandcastle example.
Update CHANGES.md.
Updates based on review.
Add conditional compilation in the vertex shader for billboard rotation.
Add custom width amd height to billboards.
Update CHANGES.md.
Updates based on review.
Update billboard Sandcastle example.
Check if fixed to ICRF matrix is defined when computing approximate LVLH.
Merge branch 'master' into approximateLVLH
If computeFixedToIcrfMatrix returns undefined, use computeTemeToPseudoFixedMatrix.
Fixes #813
Widget tweaks

1. Fixes #608 by adding a render counter and always resizing the window every 60 frames.
2. Add a `useDefaultRenderLoop` property and constructor option to `CesiumWidget` so that users can turn on/off the default render loop at will.
Merge pull request #859 from AnalyticalGraphicsInc/billboardRotation

Billboard Rotation
DynamicPositionProperty falls back to computeTemeToPseudoFixedMatrix if computeIcrfToFixedMatrix fails due to lack of EOP data.
Changes after review

1. Use frameNumber instead of a separate counter for determining when we should resize.
2. Have Viewer disable the default render loop and use it's own.
3. Expose `useDefaultRenderLoop` on Viewer as well.