util/prim: assorted fixes for u_decomposed_prims_for_vertices()

Switch to '>=' for comparisons, and it becomes obvious that the comparison for
PIPE_PRIM_QUAD_STRIP was wrong.

Add minimum vertex count check for PIPE_PRIM_LINE_LOOP.  Return 1 for
PIPE_PRIM_POLYGON with 3 vertices.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Acked-by: Zack Rusin <zackr@vmware.com>

src/gallium/auxiliary/util/u_prim.h

@@ -204,42 +204,42 @@ u_vertices_per_prim(int primitive)
 static INLINE unsigned
 u_decomposed_prims_for_vertices(int primitive, int vertices)
 {
-   switch(primitive) {
+   switch (primitive) {
    case PIPE_PRIM_POINTS:
       return vertices;
    case PIPE_PRIM_LINES:
       return vertices / 2;
    case PIPE_PRIM_LINE_LOOP:
-      return vertices;
+      return (vertices >= 2) ? vertices : 0;
    case PIPE_PRIM_LINE_STRIP:
-      return (vertices > 1) ? vertices - 1 : 0;
+      return (vertices >= 2) ? vertices - 1 : 0;
    case PIPE_PRIM_TRIANGLES:
-      return vertices /  3;
+      return vertices / 3;
    case PIPE_PRIM_TRIANGLE_STRIP:
-      return (vertices > 2) ? vertices - 2 : 0;
+      return (vertices >= 3) ? vertices - 2 : 0;
    case PIPE_PRIM_TRIANGLE_FAN:
-      return (vertices > 2) ? vertices - 2 : 0;
+      return (vertices >= 3) ? vertices - 2 : 0;
    case PIPE_PRIM_LINES_ADJACENCY:
       return vertices / 4;
    case PIPE_PRIM_LINE_STRIP_ADJACENCY:
-      return (vertices > 3) ? vertices - 3 : 0;
+      return (vertices >= 4) ? vertices - 3 : 0;
    case PIPE_PRIM_TRIANGLES_ADJACENCY:
       return vertices / 6;
    case PIPE_PRIM_TRIANGLE_STRIP_ADJACENCY:
-      return (vertices > 5) ? 1 + (vertices - 6)/2 : 0;
+      return (vertices >= 6) ? 1 + (vertices - 6) / 2 : 0;
    case PIPE_PRIM_QUADS:
       return vertices / 4;
    case PIPE_PRIM_QUAD_STRIP:
-      return (vertices > 4) ? (vertices - 2) / 2 : 0;
+      return (vertices >= 4) ? (vertices - 2) / 2 : 0;
    /* Polygons can't be decomposed
     * because the number of their vertices isn't known so
     * for them and whatever else we don't recognize just
     * return 1 if the number of vertices is greater than
-    * 3 and zero otherwise */
+    * or equal to 3 and zero otherwise */
    case PIPE_PRIM_POLYGON:
    default:
       debug_printf("Invalid decomposition primitive!\n");
-      return (vertices > 3) ? 1 : 0;
+      return (vertices >= 3) ? 1 : 0;
    }
 }