engine/terrain: assuming terrains with negative ids as missing

before that, we had a separate check for ids < 0
missing were only tiles of nonexistant chunks.
engine/terrain: ignore some diagonal influences

see the updated doc file.
mainly fixes blending for low-priority 1-tile-wide
terrain stripes, e.g. shallows.
doc: updated blendomatic file for the current algorithm
engine/terrain: simplified and clarified code

- removed unnecessary util::mod call
- split up line for shrinking the influence buffer
ci: split up configure && make and added version information dumps
ci: added python3.3 installation
convert: shallows have a lower blending mode id than ice