Backport FTOC optimization from 5.1

src/RageTypes.h

@@ -265,18 +265,31 @@ public:
  * incorrect, the first and/or last values may be biased. */
 inline unsigned char FTOC(float a)
 {
-        /* lfintf is much faster than C casts.  We don't care which way negative values
-         * are rounded, since we'll clamp them to zero below.  Be sure to truncate (not
-         * round) positive values.  The highest value that should be converted to 1 is
-         * roughly (1/256 - 0.00001); if we don't truncate, values up to (1/256 + 0.5)
-         * will be converted to 1, which is wrong. */
-        int ret = lrintf(a*256.f - 0.5f);
+	//This value is 2^52 * 1.5.
+	const double INT_MANTISSA = 6755399441055744.0;
 
-        /* Benchmarking shows that clamping here, as integers, is much faster than clamping
-         * before the conversion, as floats. */
-        if( ret<0 ) return 0;
-        else if( ret>255 ) return 255;
-        else return (unsigned char) ret;
+	/* Be sure to truncate (not round) positive values. The highest value that
+	* should be converted to 1 is roughly(1 / 256 - 0.00001); if we don't
+	* truncate, values up to (1/256 + 0.5) will be converted to 1, which is
+	* wrong. */
+	double base = double(a * 256.f - 0.5f);
+
+	/* INT_MANTISSA is chosen such that, when added to a sufficiently small
+	* double, the mantissa bits of that double can be reinterpreted as that
+	* number rounded to an integer. This is done to improve performance. */
+	base += INT_MANTISSA;
+	int ret = reinterpret_cast<int&>(base);
+
+	/* Benchmarking shows that clamping here, as integers, is much faster than clamping
+	* before the conversion, as floats. */
+	if (ret < 0) {
+		return 0;
+	}
+	if (ret > 255) {
+		return 255;
+	}
+
+	return static_cast<unsigned char>(ret);
 }
 
 /* Color type used only in vertex lists.  OpenGL expects colors in