Layer order is different in MLT output than input MVT

[springmeyer]

The layer order is unmatched between MLT output and the input MVT.

Unless there is a good reason for them to be different we should probably ensure they match.

For example in bing/4-8-5:

MVT Order:

  vector_background
  autorail
  country_region
  island
  land_cover_forest
  land_cover_grass
  populated_place
  reserve
  road
  water_feature
  water_pattern_area

MLT Order:

  water_feature
  road
  island
  land_cover_grass
  reserve
  water_pattern_area
  country_region
  land_cover_forest
  vector_background
  populated_place
  autorail

/cc @mactrem @ebrelsford