plugin interface: c++11 has log2

Signed-off-by: Tim Blechmann <tim@klingt.org>
cmake build system: supernova - compiling fftlib.c once should be enough

Signed-off-by: Tim Blechmann <tim@klingt.org>
plugins: FFT - clip window type to avoid crash

Signed-off-by: Tim Blechmann <tim@klingt.org>
plugins: FFT - cleanups

dead code is not a comment, but visual noise

Signed-off-by: Tim Blechmann <tim@klingt.org>
common: fftlib - clean up dispatching of backend

Signed-off-by: Tim Blechmann <tim@klingt.org>
common: fftlib - don't perform scaling of factor 1

Signed-off-by: Tim Blechmann <tim@klingt.org>
common: fftlib - cleanups

Signed-off-by: Tim Blechmann <tim@klingt.org>
plugins: FFT - fix invalid use of memcpy

the use of memcpy on overlapping memory regions is undefined. instead
memmove should be used.

fixes #851

Signed-off-by: Tim Blechmann <tim@klingt.org>