Add an API to RendererOptions that allows a client to enable
partial present mode, with a given maximum number of dirty
rects.

The Transaction API also has a new method, called
reset_partial_present. This method can be called when
the device is reset, or window resized etc to force a full
redraw of cached tiles.

When partial present is enabled, WR will only composite dirty
regions into the framebuffer, potentially improving GPU performance
and saving battery usage.

This is a hack / work-in-progress. It would need a better implementation before we consider merging it, but it might be good enough to start doing some experiments with to see if the basic API / concept is working correctly?

I just realized that this requires setting the reset_partial_present flag when creating a transaction, when you probably can only set it just before render()? That should be easy enough to handle in the proper implementation, if that's the case?

Yeah, Gecko will probably want to set this in render().

Yes, creating a transaction happens on compositor IPC thread. But render() is called on RenderThread.

https://hg.mozilla.org/mozilla-central/rev/79e9381b90c8