Minor clean-up of polygon offset logic. Properly compute _MRD field.
diff --git a/src/mesa/main/framebuffer.c b/src/mesa/main/framebuffer.c
index 1fd31a5..dc10d9f 100644
--- a/src/mesa/main/framebuffer.c
+++ b/src/mesa/main/framebuffer.c
@@ -65,7 +65,9 @@
       fb->_DepthMax = 0xffffffff;
    }
    fb->_DepthMaxF = (GLfloat) fb->_DepthMax;
-   fb->_MRD = 1.0;  /* Minimum resolvable depth value, for polygon offset */
+
+   /* Minimum resolvable depth value, for polygon offset */
+   fb->_MRD = 1.0 / fb->_DepthMaxF;
 }