Grayscale: avoid darkening pages with little content (#158)

Adjust black point: 0.01 -> 0.004

Author: pynicolas

imageprocessing/src/main/java/org/fairscan/imageprocessing/PostProcessing.kt

@@ -253,7 +253,7 @@ fun enhanceGrayscaleImage(img: Mat): Mat {
     val sorted = Mat()
     Core.sort(flat, sorted, Core.SORT_ASCENDING)
     val n = sorted.cols()
-    val pLow  = sorted.get(0, (n * 0.01).toInt())[0]
+    val pLow  = sorted.get(0, (n * 0.004).toInt())[0]
     val pHigh = sorted.get(0, (n * 0.99).toInt())[0]
     flat.release(); sorted.release()