Approximate sin for the reverb modulator LFO

Reverb needs to prioritize efficiency since it's expected that an app may use
multiple reverb effects simultaneously, and each individual effect may process
twice during a pipeline transition. Approximating sin helps by replacing a per-
sample libc call that we don't need to be perfectly accurate.

alc/effects/reverb.cpp

@@ -1505,14 +1505,19 @@ void ReverbPipeline::processEarly(size_t offset, const size_t samplesToDo,
 
 void Modulation::calcDelays(size_t todo)
 {
-    constexpr float mod_scale{al::numbers::pi_v<float> * 2.0f / MOD_FRACONE};
     uint idx{Index};
     const uint step{Step};
     const float depth{Depth};
     for(size_t i{0};i < todo;++i)
     {
         idx += step;
-        const float lfo{std::sin(static_cast<float>(idx&MOD_FRACMASK) * mod_scale)};
+        const float x{static_cast<float>(idx&MOD_FRACMASK) * (1.0f/MOD_FRACONE)};
+        /* Approximate sin(x*2pi). As long as it roughly fits a sinusoid shape
+         * and stays within [-1...+1], it needn't be perfect.
+         */
+        const float lfo{!(idx&(MOD_FRACONE>>1))
+            ? ((-16.0f * x * x) + (8.0f * x))
+            : ((16.0f * x * x) + (-8.0f * x) + (-16.0f * x) + 8.0f)};
         ModDelays[i] = (lfo+1.0f) * depth;
     }
     Index = idx;