Use viewport instead of initial viewport in screenshot algorithms

The screenshot algorithms reference the initial viewport when
computing framebuffer bounds. If the browser window is resized
after the document loads, this can lead to incorrect clipping.

Update the algorithms to reference the viewport instead.

Fixes #1947

Author: Sameem-baba

index.html

@@ -10926,12 +10926,12 @@ <h2>Screen capture</h2>
 
 <p>Screenshots are a mechanism for providing
  additional visual diagnostic information.
- They work by dumping a snapshot of the <a>initial viewport</a>&apos;s
+ They work by dumping a snapshot of the <a>viewport</a>&apos;s
  framebuffer as a lossless PNG image.
  It is returned to the <a>local end</a> as a Base64 encoded string.
 
 <p>WebDriver provides the <a>Take Screenshot</a> <a>command</a>
- to capture the <a>top-level browsing context</a>&apos;s <a>initial viewport</a>,
+ to capture the <a>top-level browsing context</a>&apos;s <a>viewport</a>,
  and a <a>command</a> <a>Take Element Screenshot</a>
  for doing the same with the visible region of an <a>element</a>&apos;s
  <a>bounding rectangle</a> after it has been <a>scrolled into view</a>.
@@ -10940,15 +10940,15 @@ <h2>Screen capture</h2>
  given a <a>rectangle</a>:
 
 <ol>
- <li><p>If either the <a>initial viewport</a>&apos;s width or height
+ <li><p>If either the <a>viewport</a>&apos;s width or height
   is 0 <a>CSS pixels</a>,
   return <a>error</a> with <a>error code</a> <a>unable to capture screen</a>.
 
- <li><p>Let <var>paint width</var> be the <a>initial viewport</a>&apos;s width –
+ <li><p>Let <var>paint width</var> be the <a>viewport</a>&apos;s width –
   <a>min</a>(<a>rectangle x coordinate</a>,
   <a>rectangle x coordinate</a> + <a>rectangle width dimension</a>).
 
- <li><p>Let <var>paint height</var> be the <a>initial viewport</a>&apos;s height –
+ <li><p>Let <var>paint height</var> be the <a>viewport</a>&apos;s height –
   <a>min</a>(<a>rectangle y coordinate</a>,
   <a>rectangle y coordinate</a> + <a>rectangle height dimension</a>).