Add KTX and ETC1 texture support by @Triang3l

https://github.com/xonotic/netradiant/pull/1
https://gitlab.com/xonotic/netradiant/-/issues/5

This pull request adds support for Khronos Textures to NetRadiant and Q3Map2, with OpenGL ES 2.0 formats, Ericsson Texture Compression version 1, and BGR/BGRA.

The patent-free Ericsson Texture Compression format will be used in the next version of Warsow to significantly reduce VRAM usage on mobile GPUs and the integrated GPU of Intel Broadwell.

libs/etclib.h

@@ -0,0 +1,33 @@
+// Copyright 2009 Google Inc.
+//
+// Based on the code from Android ETC1Util.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef INCLUDED_ETCLIB_H
+#define INCLUDED_ETCLIB_H
+
+#include "bytebool.h"
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+void ETC_DecodeETC1Block( const byte* in, byte* out, bool outRGBA );
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif