opus: Factor out imdct15 into a standalone component

It will be reused by the AAC decoder.

libavcodec/imdct15.h

@@ -0,0 +1,57 @@
+/*
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVCODEC_IMDCT15_H
+#define AVCODEC_IMDCT15_H
+
+#include <stddef.h>
+
+#include "avfft.h"
+
+typedef struct IMDCT15Context {
+    int fft_n;
+    int len2;
+    int len4;
+
+    FFTComplex *tmp;
+
+    FFTComplex *twiddle_exptab;
+
+    FFTComplex *exptab[6];
+
+    /**
+     * Calculate the middle half of the iMDCT
+     */
+    void (*imdct_half)(struct IMDCT15Context *s, float *dst, const float *src,
+                       ptrdiff_t src_stride, float scale);
+} IMDCT15Context;
+
+/**
+ * Init an iMDCT of the length 2 * 15 * (2^N)
+ */
+int ff_imdct15_init(IMDCT15Context **s, int N);
+
+/**
+ * Free an iMDCT.
+ */
+void ff_imdct15_uninit(IMDCT15Context **s);
+
+
+void ff_imdct15_init_aarch64(IMDCT15Context *s);
+
+#endif /* AVCODEC_IMDCT15_H */