agg changes again...

[thomas bonfort]

hi all....

I'vee finally managed to get AGG working directly on a gdImg,
completely ignoring it's alpha channel, therefore not having to switch
the alpha channel to/from gd/agg before and after rendering.

this means that it will no longer be necessary to call msAlphaAGG2GD
and msAlphaGD2AGG which is a good thing as they added some overhead
and were very hacky (except of course when using RGBA imagemode, as in
this case the alpha values in the image /are/ meaningfull)

I see two paths from this:
1/ I commit this for 5.0. we get a bit faster rendering especialy for
maps with lots of layers, but this warrants at least another beta

2/ we wait with the current solution and incorporate these changes in 5.2


I'll start with a +1 for solution no2, but people with performance
issues (Zak notably) may have another opinion on this.

thomas