I started working towards #6419. This PR introduces the logic to track where different elements of the PDF are rendered, and hooks it up to the debugger since @calixteman mentioned that it would be useful.

I'm marking this as draft because there are a few changes I need to make:

• change the various methods in canvas.js to receive the index as a param, rather than returning a function that takes the index
• clean up the "dependencies tracking", since currently it's all over the place. Ideally most of this logic should be self-contained in CanvasRecorder, so that when not recording it doesn't have a performance impact.
• improve the dependency tracking (so far I'm only tracking some of them)
• do not track extra dependencies (for example, a stroke path doesn't depend on the fill color)
• track object dependencies
• fix image dependencies tracking for transform (currently there is a .setTransform that makes it get lost)

However, I'd love to receive feedback on the direction.

Commit 1:

Add logic to track rendering area of various PDF ops

This commit is a first step towards #6419, and it can also help with
#13287. To support rendering part of a page, we will need to
first compute which ops can affect what is visible in that part of
the page.

This commit adds logic to track "group of ops" with their respective
bounding boxes. Each group eather corresponds to a single op or
to a range, and it can have dependencies earlier in the ops list that
are not contiguous to the range.

Consider the following example:

0. setFillRGBColor
1. beginText
2. showText "Hello"
3. endText
4. constructPath [...]
5. eoFill

here we have two groups: the text (range 1-3) and the path (range 4-5).
Each of them has a corresponding bounding box, and a dependency
on the op at index 0.

This tracking happens when first rendering a PDF: we wrap the canvas
with a "canvas recorder" that has the same API, but with additional
methods to mark the start/end of a group.

Commit 2:

Hook up the ops bbox logic to the pdf debugger

When using the pdf debugger, when hovering over a step now:

• it highlights the steps in the same groups
• it highlights the steps that they depend on
• it highlights on the PDF itself the bounding box

This is an example of what the debugger integration looks like (note: I couldn't figure out how to make my cursor show up in the recording 😅 I'm moving it over the steps list):

By default it doesn't show all the bounding boxes because on some PDFs it's too much noise, but if you click on the checkbox then it shows the boxes and you can click on a box to scroll into view the corresponding ops.