docs: gray out flagviz background and use wedges=on in getting started

The flag pie glyphs were drawn over the full-color rgb image, making
them hard to read against the bright tray. Use the grayscale image as
the background, consistent with labelviz and maskviz, and switch to
wedges="on" so only active flags get colored wedges.

Author: wkentaro

README.md

@@ -80,7 +80,7 @@ flags = np.column_stack(
     (masks.sum(axis=(1, 2)) < 7000, centers[:, 1] < rgb.shape[1] / 2)
 )
 flagviz = imgviz.flags2rgb(
-    rgb, flags=flags, centers=centers, flag_names=["small", "left"], wedges="all"
+    gray, flags=flags, centers=centers, flag_names=["small", "left"], wedges="on"
 )
 
 # tile instance masks

assets/getting_started.jpg

@@ -41,7 +41,7 @@
     (masks.sum(axis=(1, 2)) < 7000, centers[:, 1] < rgb.shape[1] / 2)
 )
 flagviz = imgviz.flags2rgb(
-    rgb, flags=flags, centers=centers, flag_names=["small", "left"], wedges="all"
+    gray, flags=flags, centers=centers, flag_names=["small", "left"], wedges="on"
 )
 
 # tile instance masks