The current implementation of the Edge browser creates a
WebView2Environment on first creation of an WebView2 / Edge browser
instance. On every further creation of a WebView2 instance, this
environment is used. In case the WebView2 instance is created for a
different display, i.e., within a different thread, the instantiation
fails as the WebView2Environment has been created in a different thread.

To support the creation of WebView2 instances in different displays,
this change replaces the static WebView2Environment with one environment
for every display. The WebView2 instances are created for the
environment belonging to the display for which the current instantiation
is requested. An according regression test is added.

Contributes to
https://github.com/eclipse-platform/eclipse.platform.swt/issues/1013