dix/render: consolidate window format matching code.

This code existed in 3 different forms, perhaps it should be
consolidated.

Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

dix/window.c

@@ -3660,3 +3660,16 @@ SetRootClip(ScreenPtr pScreen, Bool enable)
         WindowsRestructured();
     FlushAllOutput();
 }
+
+VisualPtr
+WindowGetVisual(WindowPtr pWin)
+{
+    ScreenPtr pScreen = pWin->drawable.pScreen;
+    VisualID vid = wVisual(pWin);
+    int i;
+
+    for (i = 0; i < pScreen->numVisuals; i++)
+        if (pScreen->visuals[i].vid == vid)
+            return &pScreen->visuals[i];
+    return 0;
+}