WebGL not available

## Expected Behavior
i try use webview in my code
but when i load path show this message
WebGL not available
also there no params in reference page
https://toga.readthedocs.io/en/latest/reference/api/widgets/webview.html

## Current Behavior

## Steps to reproduce

1. use webview to load page contain game content

2.

3.

python3.7
arrow==0.15.5
beeware==0.3.0.dev3
binaryornot==0.4.4
briefcase==0.3.0.dev7
certifi==2019.11.28
chardet==3.0.4
Click==7.0
cookiecutter==1.7.0
future==0.18.2
gbulb==0.6.1
gitdb==4.0.2
GitPython==3.1.0
idna==2.9
importlib-metadata==1.5.0
Jinja2==2.11.1
jinja2-time==0.2.0
MarkupSafe==1.1.1
pkg-resources==0.0.0
poyo==0.5.0
pycairo==1.19.1
PyGObject==3.34.0
python-dateutil==2.8.1
requests==2.23.0
six==1.14.0
smmap==3.0.1
toga==0.3.0.dev18
toga-core==0.3.0.dev18
toga-gtk==0.3.0.dev18
toml==0.10.0
travertino==0.1.2
urllib3==1.25.8
whichcraft==0.6.1
zipp==3.0.0

debian linux v10

Start with the WebView reference page at https://toga.readthedocs.io/en/latest/reference/api/widgets/webview.html and reproduce the game-content case on Debian 10 using the listed Python and Toga versions. Investigate the WebView backend's WebGL behavior and identify the missing parameters or environment limitation. Done means the cause is reproducible and either WebGL works or the limitation and supported configuration are documented.

Written by the indexing model from the issue text.