Making navigables responsible for backing store allocation will allow us
to have separate backing stores for iframes and run paint updates for
them independently, which is a step toward isolating them into separate
processes.
Another nice side effect is that now Skia backend context is ready by
the time backing stores are allocated, so we will be able to get rid of
BackingStore class in the upcoming changes and allocate PaintingSurface
directly.
With this change we save a copy of of scroll state at the time of
recording a display list, instead of actual ScrollState pointer that
could be modifed by the main thread while display list is beings
rasterized on the rendering thread, which leads to a frame painted with
inconsistent scroll state.
Fixes https://github.com/LadybirdBrowser/ladybird/issues/4288
This reduces the number of `.cpp` files that need to be recompiled when
one of the below header files changes as follows:
Painting/Command.h: 1030 -> 61
Painting/DisplayList.h: 1030 -> 60
Painting/DisplayListRecorder.h: 557 -> 59
