Store descriptor coefficients in 1 byte instead of 4

Change-Id: I26ed08a552a4452de13ed8508c3c00d68d590152
diff --git a/y2020/vision/sift/sift.fbs b/y2020/vision/sift/sift.fbs
index 3e2daaf..24fd64d 100644
--- a/y2020/vision/sift/sift.fbs
+++ b/y2020/vision/sift/sift.fbs
@@ -9,7 +9,8 @@
 
 // Represents a single feature extracted from an image.
 table Feature {
-  // Contains the descriptor data.
+  // Contains the descriptor data. OpenCV likes to represent them as floats, but
+  // they're really ubytes.
   //
   // TODO(Brian): These are scaled to be convertible to chars. Should we do
   // that to minimize storage space? Or maybe int16?
@@ -17,7 +18,7 @@
   // The size of this depends on the parameters. It is width*width*hist_bins.
   // Currently we have width=4 and hist_bins=8, which results in a size of
   // 4*4*8=128.
-  descriptor:[float];
+  descriptor:[ubyte];
 
   // Location of the keypoint.
   x:float;