Configure GDAL_CACHEMAX when running optimize-rasters

[tomalrussell]

I've been tweaking this line and bumping CACHEMAX to several GB seems to give a nice performance improvement when I have just a few large rasters to process and RAM to spare:

terracotta/terracotta/scripts/optimize_rasters.py

Line 35 in 018f963

CACHEMAX = 1024 * 1024 * 512 # 512 MB

Is there a preferred way to make this configurable? Could be a check for existing GDAL_CACHEMAX environment variable and fall back to 512MB as a safe enough default?

[dionhaefner]

@tomalrussell How's this? #338

[tomalrussell]

Looks good to me, thanks @dionhaefner