Improve distinction between color and grayscale documents (#79)

Better differentiate color and grayscale documents:
- Look for colored pixels only in the intersection of the mask and quadrilateral
- Apply a white balance (grey world) to the document
- Exclude pixels with extreme luminance
- Erode segmentation mask

evaluation/.gitignore

@@ -1,5 +1,6 @@
 /build
 /dataset/images
 /dataset/masks
+/dataset/metadata
 /python/venv
 /reports