Sensible precip. preset

- Add invisible support to mask out regions of "no rain", e.g. below a threshold set `color_mapper.low_color = rgba(0,0,0,0)`

Locate the precipitation preset and the code that configures its color_mapper. Read how thresholded regions are currently rendered, then verify that areas below the no-rain threshold become transparent while other precipitation remains visible. No file or test path is specified, so identify the relevant rendering entry point and add or run the closest available check.

Written by the indexing model from the issue text.