Fix BindGroupLayout leak in PipelineLayout default.

PipelineLayout was re-entering in NXT to create the default empty BGLs,
but forgot to remove the initial external refcount for them.

Fixing this showed an issue where it was impossible to externally
reference a RefCounted that had only internal references, fix this as
well and add a test.

src/tests/unittests/RefCountedTests.cpp

@@ -60,6 +60,24 @@ TEST(RefCounted, InternalRefKeepsAlive) {
     ASSERT_TRUE(deleted);
 }
 
+// Test that when adding an external ref from 0, an internal ref is added
+TEST(RefCounted, AddExternalRefFromZero) {
+    bool deleted = false;
+    auto test = new RCTest(&deleted);
+
+    test->ReferenceInternal();
+    test->Release();
+    ASSERT_FALSE(deleted);
+
+    // Reference adds an internal ref and release removes one
+    test->Reference();
+    test->Release();
+    ASSERT_FALSE(deleted);
+
+    test->ReleaseInternal();
+    ASSERT_TRUE(deleted);
+}
+
 // Test Ref remove internal reference when going out of scope
 TEST(Ref, EndOfScopeRemovesInternalRef) {
     bool deleted = false;