layers: LX161, validate attachment references in subpasses

For CmdClearColorAttachment, attachment ref must be in attachment
reference array of current subpass.
diff --git a/layers/draw_state.h b/layers/draw_state.h
index 86c84a0..2328936 100755
--- a/layers/draw_state.h
+++ b/layers/draw_state.h
@@ -71,6 +71,7 @@
     DRAWSTATE_BEGIN_CB_INVALID_STATE,           // Primary/Secondary CB created with mismatched FB/RP information
     DRAWSTATE_VIEWPORT_SCISSOR_MISMATCH,        // Count for viewports and scissors mismatch and/or state doesn't match count
     DRAWSTATE_INVALID_IMAGE_ASPECT,             // Image aspect is invalid for the current operation
+    DRAWSTATE_MISSING_ATTACHMENT_REFERENCE,     // Attachment reference must be present in active subpass
     DRAWSTATE_INVALID_EXTENSION,
 } DRAW_STATE_ERROR;