New library for math...

... note the swap of target_link_libraries lines in src/CMakeLists.txt,
needed to build at least on macOS, becuase FFT.h must be looked up first in
lib-math, not in lib-src/twolame

Also making a dependency cycle of SampleFormat and Dither!  But we will tolerate
that within one small library.

libraries/lib-math/SampleCount.cpp

@@ -0,0 +1,26 @@
+/**********************************************************************
+
+  Audacity: A Digital Audio Editor
+
+  @file SampleCount.cpp
+
+  Paul Licameli split from audacity/Types.h
+
+**********************************************************************/
+#include "SampleCount.h"
+
+#include <algorithm>
+#include <wx/debug.h>
+
+size_t sampleCount::as_size_t() const {
+   wxASSERT(value >= 0);
+   wxASSERT(static_cast<std::make_unsigned<type>::type>(value) <= std::numeric_limits<size_t>::max());
+   return value;
+}
+
+size_t limitSampleBufferSize( size_t bufferSize, sampleCount limit )
+{
+   return
+      std::min( sampleCount( bufferSize ), std::max( sampleCount(0), limit ) )
+         .as_size_t();
+}