New plugin: FFT, A frequency analyzer plugin

There is some more work needed:
- Keymaps are definitely not perfect, touchscreen targets are disabled due to no keymap
- There is no manual yet

Author: Delyan Kratunov 
Flyspray: FS#10065



git-svn-id: svn://svn.rockbox.org/rockbox/trunk@24587 a1c6a512-1295-4272-9138-f99709370657

apps/plugins/fft/math.c

@@ -0,0 +1,13 @@
+#include "math.h"
+
+int64_t fsqrt64(int64_t a, unsigned int fracbits)
+{
+    int64_t b = a/2 + (1 << fracbits); /* initial approximation */
+    unsigned int n;
+    const unsigned int iterations = 3; /* very rough approximation */
+
+    for (n = 0; n < iterations; ++n)
+        b = (b + (((int64_t)(a) << fracbits)/b))/2;
+
+    return b;
+}