Image given has not completed loading

Server-side canvas rendering sometimes fails with the following error:

UnhandledPromiseRejectionWarning: Error: Image given has not completed loading
    at NodeBrushTool.getImage (/opt/drawgaiden/dist/lib/canvas.js:40:25)
    at NodeBrushTool.<anonymous> (/opt/drawgaiden/node_modules/@drawgaiden/easel/lib/tools/brush.js:102:51)
    at step (/opt/drawgaiden/node_modules/@drawgaiden/easel/lib/tools/brush.js:45:23)
    at Object.next (/opt/drawgaiden/node_modules/@drawgaiden/easel/lib/tools/brush.js:26:53)
    at /opt/drawgaiden/node_modules/@drawgaiden/easel/lib/tools/brush.js:20:71
    at new Promise (<anonymous>)
    at __awaiter (/opt/drawgaiden/node_modules/@drawgaiden/easel/lib/tools/brush.js:16:12)
    at NodeBrushTool.BrushTool.drawLine (/opt/drawgaiden/node_modules/@drawgaiden/easel/lib/tools/brush.js:96:16)
    at NodeBrushTool.<anonymous> (/opt/drawgaiden/node_modules/@drawgaiden/easel/lib/tools/brush.js:172:34)
    at step (/opt/drawgaiden/node_modules/@drawgaiden/easel/lib/tools/brush.js:45:23)
    at Object.next (/opt/drawgaiden/node_modules/@drawgaiden/easel/lib/tools/brush.js:26:53)
    at /opt/drawgaiden/node_modules/@drawgaiden/easel/lib/tools/brush.js:20:71
    at new Promise (<anonymous>)
    at __awaiter (/opt/drawgaiden/node_modules/@drawgaiden/easel/lib/tools/brush.js:16:12)
    at NodeBrushTool.BrushTool.draw (/opt/drawgaiden/node_modules/@drawgaiden/easel/lib/tools/brush.js:146:16)
    at Object.<anonymous> (/opt/drawgaiden/dist/lib/canvas.js:104:42)

Brush images are loaded asynchronously but the canvas render does not wait for it to be finished, causing this error. Use node-canvas' loadImage to fix this.