Make sure MODIS landcover actually fades out at a high zoom

[clhenrick]

For reference, via Slack on 07/06/16 @mojodna said:

the fundamental problem appears that the pixels are too big (and however mapnik is doing the intersection w/ the TIFF is weird)
the difference you’re seeing between styles is a result of the `bufferSize` in `tessera.json`
it shows up at z16 when boosting it to 128
z17 at 256
and z18 at 512
...
keep in mind that the native resolution of the MODIS TIFF is slightly less than z9
which means that 1 source pixel == 1 entire tile at z17
that may be why you guys were thinking about fading it out before
so maybe it needs to be faded out more aggressively

Suggested solution by @almccon is to create separate .vrt files for each zoom from z0 - z9, (should be faded out by z10?) then set opacity on each of them.

[mojodna]

...if raster-opacity by zoom isn't working on the single layer.

[almccon]

Added the blur back, and I fade out more aggressively, so the pixelation doesn't look as bad IMHO.

[clhenrick]

Looks much better @almccon