Tighten bounds on 3d clipping.

We can only generate 6 points for even non-convex quads.
It falls from the w component crossing 0 implies we have to remove
at least one corner for each 2 we add. We can only do that twice.
4 -> 3+2 -> 2+4 -> 1+2.

BUG=None
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2627213002
Cr-Commit-Position: refs/heads/master@{#443350}

cc/base/math_util.cc

@@ -178,7 +178,7 @@ static inline bool IsNearlyTheSame(const gfx::Point3F& lhs,
 }
 
 static inline void AddVertexToClippedQuad3d(const gfx::Point3F& new_vertex,
-                                            gfx::Point3F clipped_quad[8],
+                                            gfx::Point3F clipped_quad[6],
                                             int* num_vertices_in_clipped_quad) {
   if (*num_vertices_in_clipped_quad > 0 &&
       IsNearlyTheSame(clipped_quad[*num_vertices_in_clipped_quad - 1],
@@ -309,7 +309,7 @@ gfx::Rect MathUtil::MapEnclosedRectWith2dAxisAlignedTransform(
 
 bool MathUtil::MapClippedQuad3d(const gfx::Transform& transform,
                                 const gfx::QuadF& src_quad,
-                                gfx::Point3F clipped_quad[8],
+                                gfx::Point3F clipped_quad[6],
                                 int* num_vertices_in_clipped_quad) {
   HomogeneousCoordinate h1 =
       MapHomogeneousPoint(transform, gfx::Point3F(src_quad.p1()));
@@ -370,7 +370,7 @@ bool MathUtil::MapClippedQuad3d(const gfx::Transform& transform,
                       clipped_quad[*num_vertices_in_clipped_quad - 1]))
     *num_vertices_in_clipped_quad -= 1;
 
-  DCHECK_LE(*num_vertices_in_clipped_quad, 8);
+  DCHECK_LE(*num_vertices_in_clipped_quad, 6);
   return (*num_vertices_in_clipped_quad >= 4);
 }
 

cc/base/math_util.h

@@ -189,7 +189,7 @@ class CC_EXPORT MathUtil {
   // array are valid.
   static bool MapClippedQuad3d(const gfx::Transform& transform,
                                const gfx::QuadF& src_quad,
-                               gfx::Point3F clipped_quad[8],
+                               gfx::Point3F clipped_quad[6],
                                int* num_vertices_in_clipped_quad);
 
   static gfx::RectF ComputeEnclosingRectOfVertices(const gfx::PointF vertices[],

cc/quads/draw_polygon.cc

@@ -62,7 +62,7 @@ DrawPolygon::DrawPolygon(const DrawQuad* original_ref,
       order_index_(draw_order_index),
       original_ref_(original_ref),
       is_split_(false) {
-  gfx::Point3F points[8];
+  gfx::Point3F points[6];
   int num_vertices_in_clipped_quad;
   gfx::QuadF send_quad(visible_layer_rect);