util: add rectangle helpers to u_rect.h

This begins a process of repurposing this file.  The existing usage is
as a header file for some software blit fallbacks, which should be
moved to a more appropriately named header.

src/gallium/auxiliary/util/u_rect.h

@@ -26,19 +26,68 @@
  **************************************************************************/
 
 
-/**
- * Pipe copy/fill rect helpers.
+#ifndef U_RECT_H
+#define U_RECT_H
+
+#include "pipe/p_compiler.h"
+
+struct u_rect {
+   int x0, x1;
+   int y0, y1;
+};
+
+/* Do two rectangles intersect?
  */
+static INLINE boolean
+u_rect_test_intersection(const struct u_rect *a,
+                         const struct u_rect *b)
+{
+   return (!(a->x1 < b->x0 ||
+             b->x1 < a->x0 ||
+             a->y1 < b->y0 ||
+             b->y1 < a->y0));
+}
 
+/* Find the intersection of two rectangles known to intersect.
+ */
+static INLINE void
+u_rect_find_intersection(const struct u_rect *a,
+                         struct u_rect *b)
+{
+   /* Caller should verify intersection exists before calling.
+    */
+   if (b->x0 < a->x0) b->x0 = a->x0;
+   if (b->x1 > a->x1) b->x1 = a->x1;
+   if (b->y0 < a->y0) b->y0 = a->y0;
+   if (b->y1 > a->y1) b->y1 = a->y1;
+}
 
-#ifndef U_RECT_H
-#define U_RECT_H
 
+static INLINE void
+u_rect_possible_intersection(const struct u_rect *a,
+                             struct u_rect *b)
+{
+   if (u_rect_test_intersection(a,b)) {
+      u_rect_find_intersection(a,b);
+   }
+   else {
+      b->x0 = b->x1 = b->y0 = b->y1 = 0;
+   }
+}
 
 #include "pipe/p_format.h"
 #include "util/u_pack_color.h"
 
 
+
+/**********************************************************************
+ * Pipe copy/fill rect helpers.
+ */
+
+/* These really should move to a different file:
+ */
+#include "pipe/p_format.h"
+
 extern void
 util_copy_rect(ubyte * dst, enum pipe_format format,
                unsigned dst_stride, unsigned dst_x, unsigned dst_y,