Split DisplayListBuilder into DlCanvas optimizer and DlOp recorder classes

[flar]

DisplayListBuilder grew over time from a class that implemented a developer-unfriendly stateful API into one that implemented both the developer-friendly DlCanvas API as well as its original stateful API. Over time, the stateful API was buried under a "testing only" facade.

In the meantime, the optimization features that it applies to the DlCanvas calls before it records the operations in a DlOp store are useful without the recording process and so I've been wanting to break those into 2 parts for a while with the goal of removing all stateful APIs from DisplayListBuilder (see flutter/flutter#108303).

This PR takes a major step along that direction by splitting DisplayListBuilder into essentially a convenience class that marries 2 new classes together to achieve its old functionality:

• DlCanvasToReceiver - a class that implements DlCanvas, optimizes common situations, and then sends commands to any object that implements DlOpReceiver
• DlOpRecorder - an implementation of DlOpReceiver that records the operations in a buffer from which to create a DisplayList object
• DisplayListBuilder now inherits from DlCanvasToReceiver to get the optimizations and provides it with an instance of DlOpRecorder as the receiver that it will send its results to
• Similarly, a DlCanvasToReceiver instance could be directed to an impeller:DlDispatcher to achieve a more straight-through path from the DlCanvas interface to impeller Pictures.

[flar]

It looks like a lot of new code, but really most of it came from just slicing the old DisplayListBuilder in 2 (I hoped to copy from dl_builder.{cc,h} into the 2 new files to get git to see the family resemblance and create diffs along those lines, but I don't think that git-magic was successful).

Other things that caused lots of diffs for no great reason:

• Adding a & reference tag to some of the arguments in DlOpReceiver which meant editing any implementor of that interface
• Updating lots of tests that were relying on calling the "Receiver" methods on DisplayListBuilder and getting optimizations - now that its function is split into the optimizer which only works on DlCanvas methods and the recorder which is just a direct-to-buffer stenographer they couldn't mix and match that way and had to be rewritten to the DlCanvas interface. That's actually a good thing as cleaning up all remaining uses of the Receiver APIs in the tests is on the wish list, so this PR merely prompted me to get started on that.