Implemented ST_CreateEmptyCoverage.

* ST_CreateEmptyCoverage has the same signature as ST_CreateEmptyRaster, with
  the addition of tilewidth and tileheight.
* This will create the correct number of rasters to completely cover the entire
  coverage size with regular tiling.
* The coverage may have 'remainder' rasters that aren't fully tilewidth or
  tileheight in dimension, if the width is not evenly divisible by tilewidth
  (and height/tileheight, respectively).