Properly dispose of canvas resources on graphics.destroy()

[fjwong]

Apologies in advance if work towards addressing this is already in progress, and also thank you for the effort you've poured into this library.

I had originally submited a PR for this (#495) but it seems to have been closed due to inactivity.

The recently released v3.12.0 seems to not include this fix yet, so I'm opening this issue in the hopes this helps it to be resolved quickly.

The issue:

When calling imageEditor.destroy() (which also calls graphics.destroy() under the hood), memory resources are not entirely freed up, causing eventual out of memory errors if multiple image editor instances are created, and subsequently destroyed (like when opening/closing a popup window that contains an image editor instance).

The out of memory errors are more easily reproducible on mobile devices.

The (potential) fix:

I traced the error to graphics.js' destroy method which seems to be calling Fabric.js' canvas.clear() method (which clears canvas properties and objects in it) instead of canvas.dispose().

Although I haven't examined them in deep, I believe this may be related to issues #208, #213, #375, #395, #488 and similar out of memory issues that have been reported in the past.

A workaround:

People experiencing this can potentially address this by explicitly calling canvas.dispose() when destroying an image editor instance:

// When destroying the image editor instance...
const imageEditor  = ... // Image editor instance
const canvas = imageEditor._graphics._canvas; // Get reference to fabric.Canvas object
imageEditor.destroy(); // Destroy the image editor instance
canvas.dispose(); // Finish disposing canvas resources

[lja1018]

@fjwong
Sorry for the closing of the PR. It will be released in v3.13.0.