i965: Program 2x MSAA sample positions.

There are only two sensible placements for 2x MSAA samples - and one is
the mirror image of the other.  I chose (0.25, 0.25) and (0.75, 0.75).
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>

src/mesa/drivers/dri/i965/brw_multisample_state.h

@@ -23,6 +23,17 @@
 
 #include <stdint.h>
 
+/**
+ * 1x MSAA has a single sample at the center: (0.5, 0.5) -> (0x8, 0x8).
+ *
+ * 2x MSAA sample positions are (0.25, 0.25) and (0.75, 0.75):
+ *   4 c
+ * 4 0
+ * c   1
+ */
+static const uint32_t
+brw_multisample_positions_1x_2x = 0x0088cc44;
+
 /**
  * Sample positions:
  *   2 6 a e

src/mesa/drivers/dri/i965/gen6_multisample_state.c

@@ -38,6 +38,9 @@ gen6_get_sample_position(struct gl_context *ctx,
    case 1:
       result[0] = result[1] = 0.5f;
       return;
+   case 2:
+      bits = brw_multisample_positions_1x_2x >> (8 * index);
+      break;
    case 4:
       bits = brw_multisample_positions_4x >> (8 * index);
       break;

src/mesa/drivers/dri/i965/gen8_multisample_state.c

@@ -87,10 +87,8 @@ gen8_emit_3dstate_sample_pattern(struct brw_context *brw)
    /* 4x MSAA */
    OUT_BATCH(brw_multisample_positions_4x);
 
-   /* 2x and 1x MSAA patterns
-    * XXX: need to program 2x.
-    */
-   OUT_BATCH(0x00880000);
+   /* 1x and 2x MSAA */
+   OUT_BATCH(brw_multisample_positions_1x_2x);
    ADVANCE_BATCH();
 }