Uncouple replay creation and rendering

[gberaudo]

The goal of this PR is to split the replays in 2 parts:

• classes for building instructions (typically used in web workers);
• classes for executing the instructions (typically used on the main thread).

Plan is:

• have a replay group instance for building instructions and one for executing them;
• be able to extract and replace instructions;
• physically split the builders and executor in separate classes;
• only pass information in instructions and coordinate array;

Postponed to a later phase:

• make instructions serializable (remove image creation from the styles, create and cache images in the instruction executor);
• binary transferable representation of the instructions (will be a lot of work)

[gberaudo]

@ahocevar, the creation of text instructions is now fully separated from the executor.
I will do some cosmetics cleanups and after that it should be in good shape for merging.

I will work on removing the creation of images from the styles in a different PR as it is not necessary until we have workers and I am eager to see this PR merged.

[gberaudo]

@ahocevar, the PR is now ready for review.

[gberaudo]

@ahocevar, I fixed the pixelratio issue, improved the test and fixed the mismatching executor/builder types.

I also simplified the names: Executor, Builder, ExecutorGroup, BuilderGroup. That is shorter to type and easier to remember.

The last point left is passing all states in the instructions which I will tackle tomorrow.

[ahocevar]

Great work, thanks for this! I added a FIXME about not creating the measure function with every execute() call. That can be addressed in a follow-up pull request.