ImageOverlayPlugin: Improve tile splitting behavior (#1386)

* Adjust tile splitting refine strategy, adjust non-zero error plugin behavior

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Author: gkjohnson

src/core/plugins/EnforceNonZeroErrorPlugin.js

@@ -16,15 +16,12 @@ export class EnforceNonZeroErrorPlugin {
 
 			let parent = tile.parent;
 			let depth = 1;
-
-			let targetDepth = - 1;
-			let targetError = Infinity;
 			while ( parent !== null ) {
 
-				if ( parent.geometricError !== 0 && parent.geometricError < targetError ) {
+				if ( parent.geometricError !== 0 ) {
 
-					targetError = parent.geometricError;
-					targetDepth = depth;
+					tile.geometricError = parent.geometricError * ( 2 ** - depth );
+					break;
 
 				}
 
@@ -33,14 +30,6 @@ export class EnforceNonZeroErrorPlugin {
 
 			}
 
-			// find the smallest error in the parent list to avoid grabbing artificially inflated error values
-			// for the sake of forced refinement. Then scale the error by the depth.
-			if ( targetDepth !== - 1 ) {
-
-				tile.geometricError = targetError * ( 2 ** - depth );
-
-			}
-
 		}
 
 	}

src/three/plugins/images/ImageOverlayPlugin.js

@@ -816,6 +816,11 @@ export class ImageOverlayPlugin {
 
 		} );
 
+		// force the tile "refine" mode to be set to "REPLACE" if we're splitting tiles
+		// TODO: If a tile is of type "ADD" refine and it has children then it will not be split
+		// as expected since only geometry tiles with no children are split. Instead we'd want
+		// to split this tiles geometry in addition to adding the child tiles.
+		tile.refine = 'REPLACE';
 		tile.children.push( ...children );
 
 	}