Fix logic for stretching imagery to the edge of terrain.

It wasn't correctly accounting for non-global imagery.
Merge remote-tracking branch 'origin/master' into fixImageryStretching
Make sensors render in two passes: back face and front face. Also adds ambient light to sensors.
Update based on review.
Remove unused variables.
Add tests.
Give Scene frameState, camera, and screen space camera controller initial state before rendering.
Merge branch 'master' into sensorLighting

Conflicts:
	Source/Scene/CustomSensorVolume.js
Fix after merge.
Merge pull request #970 from AnalyticalGraphicsInc/fixImageryStretching

Fix logic for stretching imagery to the edge of terrain.
Merge pull request #985 from AnalyticalGraphicsInc/sensorLighting

Sensor Lighting
Consolidate .gitignore files in the root.

This makes it easier to copy various source folders for use in other projects without accidentally ignoring items.
Merge pull request #991 from AnalyticalGraphicsInc/consolidateGitIgnore

Consolidate .gitignore files in the root.
Merge pull request #986 from AnalyticalGraphicsInc/initialSceneState

Initial Scene state.
Clone bounding sphere after creating a command instead of union with previous.
Merge branch 'master' into polylineBoundingSphere
Add test.