Error with newer versions of canvas: Object.render_png

Summary

I did a fresh install of cairo + canvas + trianglify on MacOS, then ran into an error with render_png.

This might be an issue with changes in the 2.0 release of canvas, since installing canvas@1.6.13 instead of canvas@2.5.0 seemed to resolve it.

operating system: MacOS
node: v11.14.0
npm: 6.9.2
libffii: 3.2.1
cairo: 1.16.0_2

Steps to Reproduce

The examples/save-as-png.js should be enough to reproduce this.

Error Trace

```javascript /Users/hayesall/Desktop/example/node_modules/trianglify/lib/pattern.js:54 ctx.canvas.width = opts.width; ^ TypeError: Cannot read property 'canvas' of undefined at render_canvas (/Users/hayesall/Desktop/example/node_modules/trianglify/lib/pattern.js:54:9) at Object.render_png [as png] (/Users/hayesall/Desktop/example/node_modules/trianglify/lib/pattern.js:74:12) at Object. (/Users/hayesall/Desktop/example/save-as-png.js:15:9) at Module._compile (internal/modules/cjs/loader.js:816:30) at Object.Module._extensions..js (internal/modules/cjs/loader.js:827:10) at Module.load (internal/modules/cjs/loader.js:685:32) at Function.Module._load (internal/modules/cjs/loader.js:620:12) at Function.Module.runMain (internal/modules/cjs/loader.js:877:12) at internal/main/run_main_module.js:21:11 ```

Short-term solution

Setting a hard dependency to canvas@1.6.13 might be suitable to resolving this. The following seems to resolve this error:

$ brew install cairo
$ export PKG_CONFIG_PATH="/Users/hayesall/homebrew/opt/libffi/lib/pkgconfig"
$ npm install canvas@1.6.13
$ npm install trianglify

qrohlf / trianglify