use real max glyph height

      - avoids breakage on fonts lacking an `X` glyph (#2506)
      - avoids needing to create and mutate a temporary glyph (#2516)
      - lots of minor visual changes - mostly improvements notably more correct collision boxes for lines with tall glyphs
update a few expected images in python tests
update expected grids
update expected agg images
update expected cairo images
Merge pull request #2519 from mapnik/max-glyph-height2

use real max glyph height