Fix #386: Adjust SRD::WE::is_displayed for new "displayedness"

This is sort of the "why" behind the extreme rustling that happened
when an element bounding box check was added to firefox JSONWIRE
driver, but the is_displayed method was not updated.

It also didn't hurt that their BB calc was wrong when you used
negative margins, but that's neither here nor there.  WC3 & JS
polyfill calculations are correct now.

lib/Selenium/Remote/WebElement.pm

@@ -528,6 +528,12 @@ sub get_value {
     Determine if an element is currently displayed.
     Note: This does *not* tell you an element's 'visibility' property; as it still takes up space in the DOM and is therefore considered 'displayed'.
 
+ WC3 Compatibility:
+	On JSONWire this method really only checked to see whether the element's style was display:none, or whether it was a hidden input.
+    This is because "displayedness" was pretty loosely defined until fairly late on into the process, and much grief resulted.
+    In WC3 webdriver, it additionally does a viewport check, to account for the firmer definition of "displayedness":
+    https://w3c.github.io/webdriver/#element-displayedness
+
  Output:
     BOOLEAN - Whether the element is displayed.
 
@@ -537,15 +543,29 @@ sub get_value {
 =cut
 
 sub is_displayed {
-    my ($self) = @_;
+    my ($self,$no_vp_check) = @_;
     if ($self->driver->{is_wd3} && !(grep { $self->driver->browser_name eq $_ } qw{chrome MicrosoftEdge})) {
         return 0 if $self->get_tag_name() eq 'input' && $self->get_property('type') eq 'hidden'; #hidden type inputs
+        return 0 unless $self->_is_in_viewport();
         return int($self->get_css_attribute('display') ne 'none');
     }
     my $res = { 'command' => 'isElementDisplayed', 'id' => $self->id };
     return $self->_execute_command($res);
 }
 
+sub _is_in_viewport {
+    my ($self) = @_;
+    return $self->driver->execute_script(qq{
+		var rect = arguments[0].getBoundingClientRect();
+		return (
+			rect.top >= 0 &&
+			rect.left >= 0 &&
+			rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) &&
+			rect.right <= (window.innerWidth || document.documentElement.clientWidth)
+		);
+    },{'element-6066-11e4-a52e-4f735466cecf'=> $self->{id}});
+}
+
 =head2 is_hidden
 
  Description: