make module context-aware

Issue or Feature

I've been experimenting with worker threads in node.js recently and noticed that the canvas module does not appear to be context-aware. In other words, the module will load/instantiate once, but as soon as another thread attempts to require canvas for it's own use, the following error is thrown:

Error: Module did not self-register.
    at Object.Module._extensions..node (internal/modules/cjs/loader.js:840:18)
    at Module.load (internal/modules/cjs/loader.js:666:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:606:12)
    at Function.Module._load (internal/modules/cjs/loader.js:598:3)
    at Module.require (internal/modules/cjs/loader.js:705:19)
    at require (internal/modules/cjs/helpers.js:14:16)
    at Object.<anonymous> (~/node_modules/canvas/lib/bindings.js:3:18)
    at Module._compile (internal/modules/cjs/loader.js:799:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:810:10)
    at Module.load (internal/modules/cjs/loader.js:666:32)

Steps to Reproduce

In main thread...

const {Worker} = require('worker_threads');
var worker1 = new Worker('./child-thread.js'); 
var worker2 = new Worker('./child-thread.js');
// etc.

In child-thread.js...

var Canvas = require('canvas');
var canvas = Canvas.createCanvas(200, 200);
var ctx = canvas.getContext('2d');
// etc.

References

Node.js documentation: https://nodejs.org/api/addons.html#addons_context_aware_addons Similar issue: https://github.com/schroffl/node-lzo/pull/11

Environment

• node-canvas version 2.4.1
• Mac OS 10.14.3
• node version 11.12.1