Fix transitions for *-translate properties

[jfirebaugh]

Fixes #2762, see #2762 (comment) for details.

Setting a 0-duration transition doesn't work for testing this bug, because it short circuits the code that was at fault. So in order for this to be testable in the test suite, we need to tackle mapbox/mapbox-gl-test-suite#116. I'm going to look into that next.

[jfirebaugh]

So in order for this to be testable in the test suite, we need to tackle mapbox/mapbox-gl-test-suite#116. I'm going to look into that next.

This looks like it will require significant refactoring. There are a number of internals which use Date.now() directly that need to be converted to using a passed in parameter.

• mapbox-gl-js/js/style/style_declaration.js

  Line 53 in 062f52d

  var t = Math.min((Date.now() - zh.lastIntegerZoomTime) / duration, 1);

• mapbox-gl-js/js/style/style_transition.js

  Line 54 in 8eaeebb

  var t = globalProperties.time || Date.now();

• mapbox-gl-js/js/style/style_layer.js

  Line 311 in 8eaeebb

  newTransition.loopID = animationLoop.set(newTransition.endTime - Date.now());

• mapbox-gl-js/js/render/frame_history.js

  Line 17 in 1df1466

  var now = Date.now();

• mapbox-gl-js/js/style/style.js

  Lines 250 to 257 in 7840d9c

  	if (Math.floor(zh.lastZoom) < Math.floor(z)) {
  	zh.lastIntegerZoom = Math.floor(z);
  	zh.lastIntegerZoomTime = Date.now();
  	} else if (Math.floor(zh.lastZoom) > Math.floor(z)) {
  	zh.lastIntegerZoom = Math.floor(z + 1);
  	zh.lastIntegerZoomTime = Date.now();
  	}

[jfirebaugh]

Cherry picked for v0.20.1 patch release.