Add infrastructure for glsl-unit
Use glsl-unit in WebGL map renderer
Use glsl-unit in WebGL tile layer renderer
Configure Travis to build shaders with glsl-unit
Move shader support code into ol.webgl
Reuse ol.TileRange object in forEachTileCoordParentTileRange
Avoid creating extra ol.Size object in getPixelBoundsForTileCoordAndResolution
Avoid creating ol.Coordinate objects to call getTileCoordForCoordAndResolution_
Make reverseIntersection argument mandatory
Replace getTile with getTileZXY

This massively reduces the number of temporary ol.TileCoord objects.

Previously an ol.TileCoord object was generated for every potentially
visible tile at the current zoom level and lower, every frame.  This
commit eliminates all of those.  Now new ol.TileCoord objects are only
allocated when a new tile is created.
Tidy up ol.webgl namespace
Make time-sensitive test less sensitive
Rename AnchoredElement to Overlay
Add comment about automatically generated files
Rename getTileZXY to getTile
Rename TileJSONOptions.uri to TileJSONOptions.url
Remove ol.TileRange.clone
Merge pull request #431 from twpayne/rename-anchoredelement-to-overlay

Rename AnchoredElement to Overlay
Merge pull request #425 from twpayne/glsl-unit

Use glsl-unit for WebGL shaders
Rename useTileZXY to useTile