Need to enforce AA/default gamma function post #428/r1557

[artemp]

When gamma is set on the ras_ptr, subsequent calls to reset() do not affect/reset gamma (as I assumed), and a non-default value will propagate to other features rendered on the raster surface after the gamma is modified to a value other than the default of 1.

Thanks to Oliver for reporting:

http://oliverobrien.co.uk/2010/02/prettier-maps-with-mapnik-0-7-0/

The effect is simply that symbolizers that do not allow gamma to be controlled may loose AA'ing when rendered after a polygon that was intended to have less AA by reducing gamma.

We need to enforce the default of gamma == 1 by setting agg::gamma_linear() for all symbolizers.

[artemp]

[springmeyer] fixed in r1625

[artemp]

[Ollie] Thanks Dane/Artem - I've finally got around to testing this fix (I've patched my 0.7.0 build rather than building trunk) and it works fine.

[artemp]

[springmeyer] Great Ollie, we're going to do a 0.7.1 stable release and will add this fix. Thanks for testing :)

[artemp]

[springmeyer] fixed for the 1.7.1 milestone in r1630