[test] Need SOURCE when copying image data

As cairo-test-trace does not clear the image data before reuse, using
the default OVER operator will cause differing results for each process
when inadvertently alpha blending into the shared memory region. As we
essentially want to just copy the source pixels, be explicit and set the
SOURCE operator.

test/cairo-test-trace.c

@@ -282,6 +282,7 @@ push_surface (test_runner_thread_t *thread,
     cr = cairo_create (image);
     cairo_surface_destroy (image);
 
+    cairo_set_operator (cr, CAIRO_OPERATOR_SOURCE);
     cairo_set_source_surface (cr, source, 0, 0);
     cairo_paint (cr);
     cairo_destroy (cr);