DDS: vendoring and using bcdec.h for DDS BC decompression

This can also make it easier to support BC6H in the future.

src/imageformats/bcdec.cpp

@@ -0,0 +1,11 @@
+/*
+    This file is part of the KDE project
+    SPDX-FileCopyrightText: 2022 Sergii "iOrange" Kudlai
+
+    SPDX-License-Identifier: MIT
+*/
+
+// Single-header BCn decoder used by the DDS plugin (see bcdec.h).
+
+#define BCDEC_IMPLEMENTATION
+#include "bcdec.h"