mesa: fix issues around multisample enable

multisample enable is enabled by default, however gl mandates multisample
rendering rules only apply if there's also a multisampled buffer.

src/mesa/main/state.c

@@ -288,6 +288,20 @@ update_viewport_matrix(GLcontext *ctx)
 }
 
 
+/**
+ * Update derived multisample state.
+ */
+static void
+update_multisample(GLcontext *ctx)
+{
+   ctx->Multisample._Enabled = GL_FALSE;
+   if (ctx->DrawBuffer) {
+      if (ctx->DrawBuffer->Visual.sampleBuffers)
+         ctx->Multisample._Enabled = GL_TRUE;
+   }
+}
+
+
 /**
  * Update derived color/blend/logicop state.
  */
@@ -425,6 +439,9 @@ _mesa_update_state_locked( GLcontext *ctx )
    if (new_state & (_NEW_BUFFERS | _NEW_VIEWPORT))
       update_viewport_matrix(ctx);
 
+   if (new_state & _NEW_MULTISAMPLE)
+      update_multisample( ctx );
+
    if (new_state & _NEW_COLOR)
       update_color( ctx );