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Different optimisation for software-mixer

`trunc` can resolve to a library call, while a cast-to-int doesn't
appear to. Since the mixer already does a cast-to-int when retrieving
samples, I can kill two birds with one stone.

--- a/src/Backends/Audio/SoftwareMixer.cpp

+++ b/src/Backends/Audio/SoftwareMixer.cpp

@@ -1,6 +1,6 @@

 #include "SoftwareMixer.h"

-#include <cmath>

+#include <math.h>

 #include <stddef.h>

 #include <stdio.h>

 #include <stdlib.h>

@@ -150,12 +150,15 @@

 			for (unsigned int frames_done = 0; frames_done < frames_total; ++frames_done)

 			{

+				const size_t position_integral = (size_t)sound->position;

+				const double position_fractional = sound->position - position_integral;

+

 				// Get two samples, and normalise them to 0-1

-				const float sample1 = (sound->samples[(size_t)sound->position] - 128.0f) / 128.0f;

-				const float sample2 = (sound->samples[(size_t)sound->position + 1] - 128.0f) / 128.0f;

+				const float sample1 = (sound->samples[position_integral] - 128.0f) / 128.0f;

+				const float sample2 = (sound->samples[position_integral + 1] - 128.0f) / 128.0f;

 				// Perform linear interpolation

-				const float interpolated_sample = sample1 + (sample2 - sample1) * (sound->position - std::trunc(sound->position));

+				const float interpolated_sample = sample1 + (sample2 - sample1) * position_fractional;

 				*steam_pointer++ += interpolated_sample * sound->volume_l;

 				*steam_pointer++ += interpolated_sample * sound->volume_r;