When we switched to using IOSurfaces for all resources on Mac, I didn't notice that using them on the client side was expensive because that cost was dramatically offset by other CPU and GPU power savings.

We should change this to do one-copy to an anonymous-GMB backed GLImage, as we did with IOSurface-backed GPU rasterization resources on Mac.

Some further things worth investigating:

1. It may be worth investigating if doing one-copy anonymous-GMB GLImages has a better power profile than zero-copy. This is low priority b/c SW raster is so rare.

2. We may also want to investigate service-side reuse of IOSurfaces to avoid the costs of creating, deleting, and CGLTexImageIOSurface-ing IOSurfaces.

3. Mac team also really should stop calling into these private APIs to draw scrollbars, etc -- we already have code to draw them in Skia for MacViews, we just need to hook that up.

(also point 3 in #1 will allow further sandbox lockdown, avoid messiness in impl-side-scrolling, and generally make for less sadness)

> 1. It may be worth investigating if doing one-copy anonymous-GMB GLImages has a
> better power profile than zero-copy. This is low priority b/c SW raster is so
> rare.

I think we are doing that in this case; rather than zero-copy we're using TexStorage2DImageCHROMIUM + TexSubImage2D.  Though, we're not using staging GMBs like for one-copy raster.

This should be asynchronous, but it looks like there are a lot of Flushes & WaitForTokens happening (nearly one per TexStorage2DImageCHROMIUM), so it would be interesting to see why that is.  Rather than explicit, it may be that we're just running out of transfer buffer space.  We don't scale that up very aggressively to prevent stalls.

The service side TexSubImage2D are still slow (esp. on the GT 650M on my 2012 MBP).

> 3. Mac team also really should stop calling into these private APIs to
> draw scrollbars, etc -- we already have code to draw them in Skia for
> MacViews, we just need to hook that up.

It would be interesting to understand what causes us to invalidate and re-upload all these resources.  (a) They should not be changing and (b) They're mostly out of view.