default to png8:m=h PNG format

[springmeyer]

We should consider moving from full color png to png8:m=h as the default tile image format in future releases.

png8 (reduced color, paletted png) is radically smaller. m=h triggers hextree quantization in mapnik, which is able to preserve many levels of alpha so that the image quality matches full color png. m=o (for octree) is default but only supports 3-4 levels of alpha. The reason m=h is not the default png8 mode in Mapnik is that hextree for some time has been experimental and slower. https://github.com/mapnik/mapnik/wiki/OutputFormats

But recently we've hammered on the hextree support by using it to render mapbox-streets. And there are several efforts underway to optimize hextree encoding (or to optimize paths using hextree when no alpha is needed for a given tile):

• speed up hextree encoding mapnik/mapnik#1629
• optimize png8:t=0 (no transparency mode for reduced color png) mapnik/mapnik#1557

Once the above tickets are done on the mapnik side we should consider making this change.

[springmeyer]

upstream in mapnik: mapnik/mapnik@0fb718a | mapnik/mapnik#2028