[Chrome and Safari for iPad] - Export base64 with exportImage only shows the top 30% of the stokes
**Describe the bug**
I was testing drawing something on Chrome for iPad and saw this.
After drawing a stroke, export as "png" with exportImage. The Base64 image only shows the top 30% of the strokes, but a vertical line that covers all of the canvas was draw. This is happening on the playground and in my personal project.

**To Reproduce**
1_ Open Chrome or Safari on iPad
2_ Go to https://vinoth.info/react-sketch-canvas/?path=/story/full-demo--sketch-canvas
3_ Draw something vertically across the canvas
4_ export as image (exportImage)

**Expected behavior**
Should see the entire stroke

**Actual behavior**
Only the top 30% of the stroke is visible

**Screenshots**

**Smartphone (please complete the following information):**
 - Device: iPad 5th generation
 - OS: iOS 14
 - Browser: Safari and Chrome
 - Version [e.g. 22]