Apply globalAlpha on the canvas filter source

The previous specification for canvas filters applies the globalAlpha on the filter result. The drawing model essentially says:

1. Render the shape, creating image A.
2. When the current filter is set, use it on image A to create image B.
3. Multiply the alpha component of every pixel in B by global alpha.

This leads to a unexpected behavior. When drawing a transparent shape with a drop-shadow filter, the shadow should be visible through the transparent foreground. This should be true whether the drawing is made transparent via fillStyle or via globalAlpha. This is how Chromium and Gecko behave. Following the specification to the letter however, the shadow wouldn't be visible through the foreground.

Align the standard with Chromium's and Gecko's behavior:

1. Render the shape, creating image A.
2. Multiply the alpha component of every pixel in A by global alpha, creating image B.
3. When the current filter is set, use it on image B to create image C.

source

@@ -70026,6 +70026,9 @@ console.log(pixels.data[2]);
    and line styles must be honored, and the stroke must itself also be subjected to the current
    transformation matrix.</p></li>
 
+   <li><p>Multiply the alpha component of every pixel in <var>A</var> by <code
+   data-x="concept-canvas-global-alpha">global alpha</code>.</p></li>
+
    <li>
     <p>When the <span data-x="concept-canvas-current-filter">current filter</span> is set to a
     value other than "<code data-x="">none</code>" and all the externally-defined filters it
@@ -70041,16 +70044,10 @@ console.log(pixels.data[2]);
    <li><p><span>When shadows are drawn</span>, render the shadow from image <var>B</var>,
    using the current shadow styles, creating image <var>C</var>.</p></li>
 
-   <li><p><span>When shadows are drawn</span>, multiply the alpha component of every pixel in
-   <var>C</var> by <code data-x="concept-canvas-global-alpha">global alpha</code>.</p></li>
-
    <li><p><span>When shadows are drawn</span>, composite <var>C</var> within the
    <span>clipping region</span> over the current <span>output bitmap</span> using the <span>current
    compositing and blending operator</span>.</p></li>
 
-   <li><p>Multiply the alpha component of every pixel in <var>B</var> by <code
-   data-x="concept-canvas-global-alpha">global alpha</code>.</p></li>
-
    <li><p>Composite <var>B</var> within the <span>clipping region</span> over the current
    <span>output bitmap</span> using the <span>current compositing and blending
    operator</span>.</p></li>