Swiftshader Image transparency bug

This issue demonstrates a bug I'm seeing when rendering WebGL with Swiftshader in headless mode.

Steps to reproduce

On Linux (Tested on Cent OS 7) run these two commands

node screenshot.js --headless=true
xvfb-run --server-args='-screen 0 1500x1500x16' node screenshot.js --headless=false

When Chrome is run in headless mode (headless.png) the transparent regions of the overlayed image are visible.

When using XVFB to run Chrome (xvfb.png) you will see the underlying color in the transparent regions.

Name		Name	Last commit message	Last commit date
Latest commit History 9 Commits
images		images
.gitignore		.gitignore
Makefile		Makefile
README.md		README.md
index.html		index.html
index.js		index.js
m4.js		m4.js
package.json		package.json
screenshot.js		screenshot.js
yarn.lock		yarn.lock