Introduced AGG2D_USE_FLOAT_FORMAT symbol for enabling floating-point pixel format in Agg2D, and moved include/agg_pixel_formats.h back to examples/pixel_formats.h.

git-svn-id: https://agg.svn.sourceforge.net/svnroot/agg@56 ab9de31f-a152-4fe9-8809-95f286f9ca89