No functional change. Makes the display list subsystem easier to navigate as the
major classes are in their own TUs. Also avoids importing unnecessary headers
when the previous kitchen sink header was imported. I've tried to remove all
display list related imports and start from scratch but I may have missed some
files. Minor structs and classes (like the ones in utils, ops, etc..) still
don't get their own TUs though.
There were [two](https://github.com/flutter/engine/pull/29562) [related](https://github.com/flutter/engine/pull/30484) changes being made to this subsystem that have since
landed. So I don't think I am stepping on anyones toes with the reorganization.
Happy to incorporate any work-in-progress changes being made to the this
subsystem before submitting.
* Take SUPPORT_FRACTIONAL_TRANSLATION into account when diffing layers
* Store cull_rect in device coordinates
This removes the need to update cull rect coordinates when overriding transform
and is also consistent with how Layer::needs_painting() works.
* Use original transform for culling, but overriden transform to get paint coordinates.
This is to replicate the paint process during diffing, where layers are culled first and only then after paint the matrix is overriden.
* Rewrite AddLayerBounds based on @flar's suggestions
