Fixed a layer priority problem and a bug related to empty image tiles.
Fixed an animation bug.
Merge branch 'develop' of git+ssh://github.com/mousebird/WhirlyGlobe into develop
Fixed a problem with non-power of 2 textures.
Convenience methods for starting vector tiles
Merge remote-tracking branch 'upstream/develop' into HEAD

Conflicts:
	WhirlyGlobeSrc/WhirlyGlobe-MaplyComponent/src/MaplyMultiplexTileSource.mm