(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
The approach of attaching sub-widgets to the web view widget was only
ever going to work in single-process mode, and that's not what we're
about anymore, so let's just get rid of WidgetBox so we don't have the
dead-end architecture hanging over us.
The next step here is to re-implement <input type=text> using LibWeb
primitives.
Image boxes want to know whether they are inside the visible viewport.
This is used to pause/resume animations, and to update the purgeable
memory volatility state.
Previously we would traverse the entire layout tree on every resize,
calling a helper on each ImageBox. Make those boxes register with the
frame they are interested in instead, saving us all that traversal.
This also makes it easier for other parts of the code to learn about
viewport changes in the future. :^)
