webrender reftest failures on headless

output.txt

$ git log | head -1
commit 5b83a1bc284b59c7672fb840266465c73ea6d99c
$ script/headless.py reftest
[...]
REFTEST INFO | 270 passing, 11 failing

Reftests with unexpected results:
	reftests/text/colors.yaml == reftests/text/colors-subpx.png
	reftests/text/colors.yaml == reftests/text/colors-subpx.png
	reftests/text/border-radius.yaml == reftests/text/border-radius-subpx.png
	reftests/text/text-masking.yaml == reftests/text/text-masking-subpx.png
	reftests/text/subpixel-rotate.yaml == reftests/text/subpixel-rotate.png
	reftests/text/subpixel-scale.yaml == reftests/text/subpixel-scale.png
	reftests/text/subpixel-skew.yaml == reftests/text/subpixel-skew.png
	reftests/text/clipped-transform.yaml == reftests/text/clipped-transform.png
	reftests/text/blurred-shadow-local-clip-rect.yaml == reftests/text/blurred-shadow-local-clip-rect-ref.png
	reftests/text/shadow-transforms.yaml == reftests/text/shadow-transforms.png
	reftests/text/raster-space.yaml == reftests/text/raster-space.png
Traceback (most recent call last):
  File "script/headless.py", line 75, in <module>
    subprocess.check_call(['../target/release/wrench', '--no-scissor', '-h'] + sys.argv[1:])
  File "/usr/lib/python3.6/subprocess.py", line 291, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['../target/release/wrench', '--no-scissor', '-h', 'reftest']' returned non-zero exit status 11.