A struct that is reponsible for delaying frame requests until all new canvas images
for a particular “update the rendering” call in the ScriptThread have been
sent to WebRender.
A Painter is responsible for all of the painting to a particular RenderingContext.
This holds all of the WebRender specific data structures and state necessary for painting
and handling events that happen to WebViews that use a particular RenderingContext.
Notable is that a Painter might be responsible for painting more than a single
[WebView] as long as they share the same RenderingContext.
The paint status of a particular pipeline in a Painter. This is used to trigger metrics
in script (via the constellation) when display lists are received.