Element screenshots incorrect when page has been scrolled

[jorenko]

When the page has been scrolled, the element coordinates returned by NeedleWebElementMixin.get_dimensions() are relative to the top-left of the page, but the screenshot that get_screenshot() crops starts at the current scroll position. This results in the returned screenshot being of either the wrong part of the page or mostly/all blank, depending on the browser. To fix this, the current scroll position needs to be subtracted from the position, like so:

diff --git a/home/jjurack/.local/share/virtualenvs/test_web-6ViYfjms/lib/python3.6/site-packages/needle/driver-orig.py b/home/jjurack/.local/share/virtualenvs/test_web-6ViYfjms/lib/python3.6/site-packages/needle/driver.py
index cd10b49..eceafe5 100644
--- a/home/jjurack/.local/share/virtualenvs/test_web-6ViYfjms/lib/python3.6/site-packages/needle/driver-orig.py
+++ b/home/jjurack/.local/share/virtualenvs/test_web-6ViYfjms/lib/python3.6/site-packages/needle/driver.py
@@ -63,8 +63,8 @@ class NeedleWebElementMixin(object):
         d = self.get_dimensions()
         
         # Cast values to int in order for _ImageCrop not to break
-        d['left'] = int(d['left'])
-        d['top'] = int(d['top'])
+        d['left'] = int(d['left']) - self._parent.execute_script('return window.scrollX')
+        d['top'] = int(d['top']) - self._parent.execute_script('return window.scrollY')
         d['width'] = int(d['width'])
         d['height'] = int(d['height'])