TerrainExtension: Fix cover dirty flag ambiguity

When rendering a picking pass, the pickingFbo is updated every time.
The dirty flag refers to the rendering fbo.
Previously the dirty flag was updated on picking passes too which
sometimes led to the rendering fbo not being updated when it was still
dirty.

Author: flostellbrink

modules/extensions/src/terrain/terrain-effect.ts

@@ -202,7 +202,12 @@ export class TerrainEffect implements Effect {
             devicePixelRatio: 1
           }
         });
-        terrainCover.isDirty = false;
+
+        if (!this.isPicking) {
+          // IsDirty refers to the normal fbo, not the picking fbo.
+          // Only mark it as not dirty if the normal fbo was updated.
+          terrainCover.isDirty = false;
+        }
       }
     } catch (err) {
       terrainLayer.raiseError(err as Error, `Error rendering terrain cover ${terrainCover.id}`);