agg changes

[thomas bonfort]

hi devs,
I've commited a few changes to the agg rendering logic, which greatly
increase performance when drawing many shapes. what's changed is that the
imageObj now holds a pointer to the rendering object and all it's
dependencies, instead of recreating them nearly from scratch each time a
shape is drawn.
on the "ntdb_roads_50" from a recent osgeo-discuss post
before: 1.1seconds
after: 0.5 seconds

this is still much more than what gd does, but getting closer (that
particular test file is probably a worst case scenario when comparing
rendering times: many tiny line segments that just show up as a pixel with
gd, but all have to be antialiased in agg)

all this to say that I don't think there should be much breakage arising
from this, but I'd appreciate if all of you who are set up to test agg could
give the new version a run and report if anything weird is happening.

thanks,
thomas
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