Geocoder improvements

1. Clear input field when the home button is used.
2. Change input to type `search` which provides an `x` button for easy clearing.
Geocoder style tweaks
Drop MOZ_ WebGL extension prefixes to avoid warnings in Firefox
Changed extensions to use undefined when not available
Added failIfMajorPerformanceCaveat
Workaround tests until failIfMajorPerformanceCaveat is returned by getContextAttributes
Undefined tweak
Added allowTextureFilterAnisotropic
Update CHANGES.md
Added maximumScreenSpaceError
Initial submit for pixel offset scaling by distance.
Added pixelOffsetByDistance to labels.  Added Sandcastle example to demonstrate a usecase.
CSS tweak.
Updated Dynamic layer to support time dynamic Label and Billboard PixelOffsetByDistance.
Refactored PixelOffsetByDistance to instead be PixelOffsetScaleByDistance.  This required the addition of a private member, Billboard._translate that is used by labels to separate glyph translation from pixelOffset settings.
Updated Billboard.computeScreenSpacePosition to use _translate in addition to pixelOffset for screen space positioning.  Updated LabelCollection Unit tests.
Updated DynamicScene to use the pixelOffsetScaleByDistance over the former pixelOffsetByDistance.  Added BillboardCollection, DynamicLabel, and DynamicBillboard unit tests.
Updated Changes.MD.  Minor cleanup.
Added Billboard.setPixelOffsetScaleByDistance SandCastle example.  Minor tweak to Label's pixelOffsetScaleByDistance sandcastle example.
Adding dynamic vector display support.