Pica/Texture: Move part of ETC1 decoding to new file and cleanups

src/video_core/texture/etc1.h

@@ -0,0 +1,16 @@
+// Copyright 2017 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include "common/common_types.h"
+#include "common/vector_math.h"
+
+namespace Pica {
+namespace Texture {
+
+Math::Vec3<u8> SampleETC1Subtile(u64 value, unsigned int x, unsigned int y);
+
+} // namespace Texture
+} // namespace Pica