Let style changes that only rebuild compatible accumulated visual
contexts avoid marking the display list dirty. This lets transform
and nonzero opacity updates send visual context tree updates without
recording a new display list.
Keep repainting changes that affect display-list contents or can change
visual context tree compatibility, including zero-crossing opacity,
transform invertibility crossings, background-attachment, clipping,
mix-blend-mode, and perspective. Schedule accumulated visual context
updates for animations independently of repaint so animated
transform/effect updates keep reaching the document.
Cover compatible visual context reuse, incompatible tree shapes, and the
display-list invalidation cases with focused LibWeb tests.
Let accumulated visual context updates keep the previous tree version
when rebuilt with the same shape. Display lists reference visual
context tree versions, so keep compositor-only updates on the old
version unless the tree structure changes.
Add coverage for version reuse and incompatible tree shapes.
