moving instead of copying mapnik::glyph_info

Conflicts:
	include/mapnik/text/dummy_shaper.hpp
	include/mapnik/text/harfbuzz_shaper.hpp
	include/mapnik/text/icu_shaper.hpp
premultiply R,G,B channels after applying filters
Merge branch 'master' into unscaled-glyphs

Conflicts:
	include/mapnik/text/dummy_shaper.hpp
	include/mapnik/text/harfbuzz_shaper.hpp
	include/mapnik/text/icu_shaper.hpp