fix(EraserBrush): use rendered objects for pattern

[ShaMan123]

Previously the eraser brush used the _objects attribute of canvas to render necessary pattern for the erasing effect.
Fixed to use the _objectsToRender attribute. This way if objects are rendered in a custom way (stack order changes or something) the eraser plays nicely with it.

[github-actions]

Code Coverage Summary

> fabric@5.1.0 coverage:report
> nyc report --reporter=lcov --reporter=text

-----------|---------|----------|---------|---------|-----------------------------------------------
File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                             
-----------|---------|----------|---------|---------|-----------------------------------------------
All files  |    83.1 |    76.01 |   85.69 |   82.83 |                                               
 fabric.js |    83.1 |    76.01 |   85.69 |   82.83 | ...,30850,30924,30935-31000,31123,31222,31458 
-----------|---------|----------|---------|---------|-----------------------------------------------

[asturur]

i still have no idea what erarserbrush does and how. And i won't for long probably, so i approve those PR in trust when they do not modify other fabricjs files

[ShaMan123]

Thanks!
It's a minor change to align with the changes we made to Canvas#_objectsToRender
I am working on visual tests for the eraser. Will finish soon.
I am facing a weird edge case that I'm struggling to get to the bottom of and you have made me understand the necessity of tests. I am in fear of the eraser breaking due to major changes. Tests will ease my mind.