libhwcomposer: Enhance mixed mode composition.

While marking layers for cache, neglect the z-order when
possible, to improve the scope of mixed mode usage.
It is safe to neglect the z-order if a static layer
doesn't have any overlapping region with updating layers
in between static layer & contiguous static layers batch.
- if above layer doesn't have any overlapping with updating
  layer in middle, push it to the batch.
- If above case fails, but layers below(existing batch) does not
  have any overlapping region with the updating layer, bring the
  batch to top and modify fb-zorder accordingly.

Change-Id: I4882a750d0a9a2e11272fcbb146202160673d04f

libhwcomposer/hwc_utils.cpp

@@ -923,6 +923,13 @@ void calculate_crop_rects(hwc_rect_t& crop, hwc_rect_t& dst,
     crop_b -= crop_h * bottomCutRatio;
 }
 
+bool areLayersIntersecting(const hwc_layer_1_t* layer1,
+        const hwc_layer_1_t* layer2) {
+    hwc_rect_t irect = getIntersection(layer1->displayFrame,
+            layer2->displayFrame);
+    return isValidRect(irect);
+}
+
 bool isValidRect(const hwc_rect& rect)
 {
    return ((rect.bottom > rect.top) && (rect.right > rect.left)) ;