remove background color from polygon-pattern files

Remove background color from polygon-pattern files as was already the case for forest. This makes changing landuse colors easier. No visual changes with one exception, see below.

Following patterns are affected:
* beach
* glacier
* grave yard
* mud
* orchard
* quarry
* scrub
* vineyard (is now brighter for lower zoom levels)
* zoo

Following patterns are NOT affected:
* allotments (background cannot be removed without destroying the pattern)
* military danger area (insufficient data for testing)
* military landuse (due to transparency)