feat: support src collaborators (#5114)

* feat: support avatarURLfor collaborators

* fix

* better avatars :)

* use position fixed for tooltips so it renders correctly when offsets updated

* update docs

* Update src/excalidraw-app/collab/CollabWrapper.tsx

* rename avatarUrl to src

src/types.ts

@@ -45,6 +45,9 @@ export type Collaborator = {
     background: string;
     stroke: string;
   };
+  // The url of the collaborator's avatar, defaults to username intials
+  // if not present
+  src?: string;
 };
 
 export type DataURL = string & { _brand: "DataURL" };