[cairo] [xlib] Use server-side gradients.

[Carl Worth]

Excerpts from ickle's message of Fri Jul 31 06:42:09 -0700 2009:
>     [xlib] Use server-side gradients.
>     
>     We can offload creation of gradients to server that support RENDER 0.10
>     and later. This greatly reduces the amount of traffic we need to send over
>     our display connection as the gradient patterns are much smaller than the
>     full image. Even if the server fallbacks to using pixman, performance
>     should be improved by the reduced transport overhead. Furthermore this is a
>     requisite to enable hardware accelerated gradients with the xlib backend.
>     
>     Running cairo-perf-trace on tiny, Celeron/i915:
>     
>       before: firefox-20090601 211.585
>        after: firefox-20090601 270.939
>     
>     and on tiger, CoreDuo/nvidia:
>     
>       before: firefox-20090601 70.143
>        after: firefox-20090601 87.326
>     
>     where linear gradients are used extensively throughout the GTK+ theme.
>     Not quite the result I was expecting!
...
>     Before 1.10, we may choose to disable server-side gradients for the
>     current crop of Xorg servers, similar to the extended repeat modes.

Shouldn't we have discussions like this on the mailing list, rather
than the commit messages?

And perhaps, even before the code lands?

I'm just a bit nervous about forgetting this now and accidentally

> +Backend improvements:
> +
> +  xlib
> +
> +     Server-side gradients. The theory is that we can offload computation
> +     of gradients to the GPU and avoid pushing large images over the
> +     connection. Even if the driver has to fallback and use pixman to render
> +     a temporary source, it should be able to do so in a more efficient manner
> +     than Cairo itself. However, cairo-perf suggests otherwise:

This change to NEWS helps a bit, I suppose. But still, let's not skip
the mailing list, OK? (Or if I missed a message, I'll just blame my
old MUA and you can ignore me.)

-Carl
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