RenderRectFilledRangeH() can't use cheap acosf() approximation for now. (#1296)

imgui_draw.cpp

@@ -2337,7 +2337,8 @@ static inline float ImAcos01(float x)
 {
     if (x <= 0.0f) return IM_PI * 0.5f;
     if (x >= 1.0f) return 0.0f;
-    return (-0.69813170079773212f * x * x - 0.87266462599716477f) * x + 1.5707963267948966f; // Cheap approximation, enough for what we do.
+    return acosf(x);
+    //return (-0.69813170079773212f * x * x - 0.87266462599716477f) * x + 1.5707963267948966f; // Cheap approximation, may be enough for what we do.
 }
 
 // FIXME: Cleanup and move code to ImDrawList.