Compositor: Add note that bilateral blur is not really correct

The code does stupid box filter which doesn't give nice results and doesn't
match old compositor at all. Need some better gaussian weighting here. Will
look into it later.

source/blender/compositor/operations/COM_BilateralBlurOperation.cpp

@@ -65,6 +65,10 @@ void BilateralBlurOperation::executePixel(float output[4], int x, int y, void *d
 
 	zero_v4(blurColor);
 	blurDivider = 0.0f;
+	/* TODO(sergey): This isn't really good bilateral filter, it should be
+	 * using gaussian bell for weights. Also sigma_color doesn't seem to be
+	 * used correct at all.
+	 */
 	for (int yi = miny; yi < maxy; yi += QualityStepHelper::getStep()) {
 		for (int xi = minx; xi < maxx; xi += QualityStepHelper::getStep()) {
 			// read determinator