Hey MCR1. Excellent work, here's some comments so far.

8	+    CompOption::Vector o(0);
9	     dndWindow->ungrabNotify ();
10	 
11	+    screen->handleCompizEvent ("expo", "start_viewport_switch", o);

I believe there is a constant somewhere, either as a static member of CompOption or just as a global called "noOptions". I'd suggest using that.

43	-	<_long>Warp and resize windows to fit an imaginary grid.</_long>
44	+	<_long>Warp and resize windows to fit an imaginary grid</_long>
45	 	<category>Window Management</category>
46	 	<deps>
47	 	    <requirement>
48	@@ -83,7 +83,7 @@
49	 			<_long>Window resize action</_long>
50	 			<option name="top_left_corner_action" type="int">
51	 			    <_short>Upper Left Corner</_short>
52	-			    <_long>Action to be performed when window is dropped on the top left corner</_long>
53	+			    <_long>Action to be performed when window is dropped on the top left corner.</_long>

Try to avoid mixing formatting changes with substantive changes.

319	-		 * computing what the 33% and 66% offsets would be
320	-		 */
321	+		     * computing what the 33% and 66% offsets would be
322	+		     */

I think that's probably an undesired formatting change.

Statements like this:

377	+		if (where & GridLeft || where & GridRight ||
378	+		    where & GridTop || where & GridBottom)

And this:

308	+	    (where & ~(GridMaximize) ||
309	+	     (where & ~(GridLeft | GridRight | GridTop | GridBottom) &&

Can be combined, eg

unsigned int VerticalGrid = GridLeft | GridRight;
unsigned int HorizontalGrid = GridTop | GridBottom;
unsigned int ConstrainedGrid = VerticalGrid | HorizontalGrid | GridMaximize;

if (where & ~ConstrinedGrid) 

...

if (where & VerticalGrid)
{
}
else if (where & HorizontalGrid)
{
}

530	+    else if (!gw->isGridResized &&
531	+	     gw->isGridHorzMaximized &&
532	+	     !gw->isGridVertMaximized)
533	+    {
534	+	/* Window has been horizontally maximized by grid. We only need
535	+	 * to restore Y and height - core handles X and width. */
536	+	if (gw->sizeHintsFlags)
537	+	    gw->window->sizeHints ().flags |= gw->sizeHintsFlags;
538	+	xwcm |=  CWY | CWHeight;
539	+    }
540	+    else if (!gw->isGridResized &&
541	+	     !gw->isGridHorzMaximized &&
542	+	     gw->isGridVertMaximized)
543	     {
544	 	/* Window has been vertically maximized by grid. We only need
545	-	 * to restore the X and width - core handles Y and height. */
546	+	 * to restore X and width - core handles Y and height. */
547	 	if (gw->sizeHintsFlags)
548	 	    gw->window->sizeHints ().flags |= gw->sizeHintsFlags;
549	 	xwcm |= CWX | CWWidth;
550	     }
551	-
552	-    else if (gw->isGridResized && !gw->isGridSemiMaximized)
553	-	/* Window is just gridded (top, bottom, center, corners). We
554	-	 * need to handle everything. */
555	+    else if (gw->isGridResized &&
556	+	     !gw->isGridHorzMaximized &&
557	+	     !gw->isGridVertMaximized)
558	+	/* Window is just gridded (center, corners).
559	+	 * We need to handle everything. */

It might make sense to nest these blocks.

if (!isGridResized)
{
    assert (isGridHorzMaximized != isGridVertMaximized);

    if (isGridHorzMaximized)
    {
        ...
    }
    else if (isGridVertMaximized)
    {
        ...
    }
}
else
{
    assert (!isGridHorzMaximized);
    assert (!isGridVertMaximized);
    ...
}

Though the use of all these boolean flags has made me wonder if it would be better to use an enum instead:

enum CurrentGridState
{
    None = 0,
    Resized = 1,
    VerticallyMaximized = 2,
    HorizontallyMaximized = 3
};