wgl: do not disable error-dialogs by default

We don't know if an application developer wants error-dialog by default
or not, even when using a Mesa OpenGL driver.

So let's not assume this is a good thing, and instead make an option for
this behavior that we can enable for CI testing.

Reviewed-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15485>

docs/envvars.rst

@@ -235,6 +235,9 @@ on Windows.
 :envvar:`WGL_FORCE_MSAA`
    if set to a positive value, specifies the number of MSAA samples to
    force when choosing the display configuration.
+:envvar:`WGL_DISABLE_ERROR_DIALOGS`
+   if set to 1, true or yes, disables Win32 error dialogs. Useful for
+   automated test-runs.
 
 Intel driver environment variables
 ----------------------------------------------------