Draw adjustment thumbnails as a half-filled circle. (#4628)

The contrast glyph put a smaller semicircle inside the ring, which read as concentric rings on a layer thumbnail. It is now one circle with a filled half. (#4561)

Co-authored-by: Brandon Thomas <[email protected]>

assets/icons/contrast.svg

@@ -10,5 +10,5 @@
   stroke-linejoin="round"
 >
   <circle cx="12" cy="12" r="10" />
-  <path d="M12 18a6 6 0 0 0 0-12v12z" />
+  <path d="M12 2a10 10 0 0 0 0 20z" fill="currentColor" stroke="none" />
 </svg>