Fix ASTextNode truncated size calculation ignoring attributes in the last line, by syncing the truncationString's attributes with the primary attributedString.
This should not directly affect more complex apps that don't use the ASBasicImageDownloader.
Also disables the default-on text placeholders, as they churn memory during the measurement pass.
These were intended to be written with pure layers (without using backing stores), so I don't
think it is a reasonable default-on behavior until that is fixed.
1) Set the ascender/descender of an ASTextNode when the attributeString is set. Previously ascender/descender were only being computed in `setValuesFromLayoutable` and only when the attribute string was not nil. May make sense to remove the computation from `setValuesFromLayoutable` entirely.
2) Remove ability to allow different children of a stack spec to aling to different baselines. This wasn't working before and I'm not convinced it is possible to do properly/useful enough to invest the time.
3) Have all stack spec run `ASStackBaselinePositionedLayout::compute` to compute the stack's ascender and descender. Even if the stack isn't aligning its children to a baseline, the stack itself may be a child of another stack that IS aligning to a baseline.
- Both ASDisplayNode and ASLayoutNode conforms to this protocol.
- ASDisplayNode can be embeded directly into layout graph.
- Eliminate ASCompositeNode.
- Fix ASStaticSizeDisplayNode not recpect min constrained size.
- Updated tests.
- Introduce ASLayoutNode and its subclasses.
- ASDisplayNode measures its ASLayoutNode and cache the result (ASLayout). Calculated size and position of each subnode can be retrieved from the calculated layout.
- Custom nodes need to override -layoutNodeThatFits:(CGSize) instead of -calculateSizeThatFits:(CGSize).
- Custom nodes do not need to layout its subnodes (in -layout:) anymore. ASDisplayNode can handle the job most of the time, by walking through its layout tree.
- ASCompositeNode is used to embed (display) subnodes to a node's layout. That way, each subnode will also be measured while the parent node is measuring. And the parent node knows where its subnodes are within its layout.
