Factored out common renderer fields.

The idea here is that much of the code in the renderers and the
various `process_*` rendering methods is shared, but this sharing
is obfuscated by the lack of sharing of renderer fields and
methods.

This is the first step in trying to factor out commonalities in
order to simplify the implementation of the renderers and make
them easier to understand and modify.
Factor common fields out of cairo renderer.

This involved making the freetype engine stack allocated and
accessible via a `shared_ptr` because `FT_Face` elements stored
in it are referenced from cairo structures which persist beyond
the lifetime of the `cairo_renderer` object.

Hopefully this can be resolved in a future commit.
Factored out common fields from grid renderer.
Factored common renderer code from SVG renderer.
Factored out common code for building symbolizer.

There's a lot of code common between symbolizers which do things
like layout, setup, etc... So it seems sensible to extract the
common operations & loops into a single place which is
independent of the actual renderer.

I've started doing this with C++11 lambdas, although that might
not be the best implementation long term. Should be easy enough
to change, though.
Factored out common implementation of point symbolizer.
Factored out common implementation of raster symbolizer.
Factored out markers symbolizer logic.

It's not ideal - there's still a bunch of "dispatch" objects from
which the common elements could be factored for greater
readability.
Factored out common implementation of polygon symbolizer.
Merge remote-tracking branch 'upstream/expr-v2' into mla-expr-v2-renderer-common
Merge remote-tracking branch 'upstream/expr-v2' into mla-expr-v2-renderer-common
Fixed compile error due to typo in svg_renderer.cpp
fix : use 'template' keyword to treat 'set' as a dependent template name error (clang)
Merge branch 'MapQuest-mla-expr-v2-renderer-common' into expr-v2
`scale_factor_` now in `common_`.
Merge branch 'MapQuest-mla-expr-v2-renderer-common' into expr-v2