`toRaster` is slow even for very small images

[metasim]

From @metasim on September 25, 2017 19:41

Tests in RasterFrameSpec and ExplodeSpec show this.

Copied from original issue: s22s/raster-frames#27

[metasim]

"Stack Trace";"Sample Count";"Percentage(%)"
"geotrellis.raster.merge.SinglebandTileMergeMethods$class.merge(SinglebandTileMergeMethods, Extent, Extent, Tile, ResampleMethod)";"1,277";"61.721"
"   geotrellis.raster.package$withTileMethods.merge(Extent, Extent, Tile, ResampleMethod)";"1,277";"61.721"
"      geotrellis.raster.package$withTileMethods.merge(Extent, Extent, Object, ResampleMethod)";"1,277";"61.721"
"         geotrellis.spark.tiling.CutTiles$$anonfun$apply$1$$anonfun$apply$2.apply(Tuple2)";"1,277";"61.721"
"            geotrellis.spark.tiling.CutTiles$$anonfun$apply$1$$anonfun$apply$2.apply(Object)";"1,277";"61.721"