Only call dashStyle when strokeDashstyle is set

[ahocevar]

Currently, with every feature that has a stroke symbolizer set, the dashStyle method will be called to determine the dash style. This is not only relatively expensive, it also requires the strokeDashstyle to have a non-null default in the new Symbolizer classes.

The attached patch removes strokeDasharray from the renderer's defaultSymbolizer, adds an additional check in the renderers to only call dashStyle when the strokeDashstyle symbolizer property is set, and sets the strokeDashstyle default to null in the new Symbolizer classes.

Tests still pass, thanks for any review.

[tschaub]

Nice! Thanks for fixing this up.

[ahocevar]

(In [10621]) Only call dashStyle when strokeDashstyle is set. r=tschaub (closes #2768)