Add test for offscreen_canvas_is_valid().

Author: juj

test/offscreen_canvas_is_valid.cpp

@@ -0,0 +1,12 @@
+#include "lib_webgpu.h"
+#include <assert.h>
+
+int main()
+{
+  assert(!offscreen_canvas_is_valid(0));
+  assert(!offscreen_canvas_is_valid(42));
+  canvas_transfer_control_to_offscreen("canvas", 42);
+  assert(offscreen_canvas_is_valid(42));
+
+  EM_ASM(window.close());
+}