Initial commit for issue #1076.  Added translucencyByDistance to billboard
and incorporated into Label.  SandCastle examples added for both label and
billboard fading based on camera distance.

Signed-off-by: Alex Wood <abwood@gmail.com>
Merge remote-tracking branch 'agi/master' into issue1076
Fixed typo in Billboard asset name.

Signed-off-by: Alex Wood <abwood@gmail.com>
Fixed bug that would occur if some billboards in a collection did not use
scale/translucencyByDistance, their default values would cause issues.
For translucency and scale, we should default to 1.0 which results in an
identity transformation (e.g., scale by 1).

Signed-off-by: Alex Wood <abwood@gmail.com>
Added unit tests for Billboard.translucencyByDistance and
Label.translucencyByDistance.  Cleanup and fixes related to unit tests.

Signed-off-by: Alex Wood <abwood@gmail.com>
Added unit tests for picking labels and billboards that use
translucencyByDistance.  Picking fixes for translucencyByDistance.  Updated
Changes.MD.

Signed-off-by: Alex Wood <abwood@gmail.com>
Minor documentation fixes.

Signed-off-by: Alex Wood <abwood@gmail.com>
Add translucencyByDistance support to DynamicScene.
Merge remote-tracking branch 'agi/master' into issue1076

Conflicts:
	CHANGES.md
Clipping billboards that have scale or translucency of 0.0 in the vertex
shader.  Minor cleanup in unit tests and Sandcastle example.

Signed-off-by: Alex Wood <abwood@gmail.com>
Merge remote-tracking branch 'agi/master' into issue1076

Conflicts:
	Source/Scene/Billboard.js
	Source/Scene/Label.js
	Specs/Scene/LabelCollectionSpec.js
Explicit assignment to zero for Billboard vertex shader clipping of
vertex with scale or translucency equal to 0.

Signed-off-by: Alex Wood <abwood@gmail.com>
Revised presentation in Label translucency Sandcastle example.

Signed-off-by: Alex Wood <abwood@gmail.com>
Merge remote-tracking branch 'agi/master' into issue1076
Merge pull request #1213 from abwood/issue1076

Issue1076