intel: Update Mesa state before span setup in glReadPixels.

We could have mapped the wrong set of draw buffers.  Noticed while looking
into a DRI2 glean ReadPixels issue.

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

@@ -0,0 +1 @@
+../intel/intel_pixel_read.c

src/mesa/drivers/dri/intel/intel_pixel.c

@@ -424,10 +424,8 @@ intelInitPixelFuncs(struct dd_function_table *functions)
       functions->Bitmap = intelBitmap;
       functions->CopyPixels = intelCopyPixels;
       functions->DrawPixels = intelDrawPixels;
-#ifdef I915
-      functions->ReadPixels = intelReadPixels;
-#endif
    }
+   functions->ReadPixels = intelReadPixels;
 }
 
 void

src/mesa/drivers/dri/intel/intel_pixel_read.c

@@ -291,6 +291,7 @@ intelReadPixels(GLcontext * ctx,
 
    intelFlush(ctx);
 
+#ifdef I915
    if (do_blit_readpixels
        (ctx, x, y, width, height, format, type, pack, pixels))
       return;
@@ -298,9 +299,19 @@ intelReadPixels(GLcontext * ctx,
    if (do_texture_readpixels
        (ctx, x, y, width, height, format, type, pack, pixels))
       return;
+#endif
 
    if (INTEL_DEBUG & DEBUG_PIXEL)
       _mesa_printf("%s: fallback to swrast\n", __FUNCTION__);
 
+   /* Update Mesa state before calling down into _swrast_ReadPixels, as
+    * the spans code requires the computed buffer states to be up to date,
+    * but _swrast_ReadPixels only updates Mesa state after setting up
+    * the spans code.
+    */
+
+   if (ctx->NewState)
+      _mesa_update_state(ctx);
+
    _swrast_ReadPixels(ctx, x, y, width, height, format, type, pack, pixels);
 }