Use image cache to stop leaking images (#29452)

This PR fixes several possible memory leaks due to loading images in
markdown files and the image viewer, using the new image cache APIs

TODO: 
- [x] Ensure this didn't break rendering in any of the affected
components.

Release Notes:

- Fixed several image related memory leaks

crates/zed/src/zed.rs

@@ -26,8 +26,8 @@ use git_ui::project_diff::ProjectDiffToolbar;
 use gpui::{
     Action, App, AppContext as _, AsyncWindowContext, Context, DismissEvent, Element, Entity,
     Focusable, KeyBinding, ParentElement, PathPromptOptions, PromptLevel, ReadGlobal, SharedString,
-    Styled, Task, TitlebarOptions, UpdateGlobal, Window, WindowKind, WindowOptions, actions, point,
-    px,
+    Styled, Task, TitlebarOptions, UpdateGlobal, Window, WindowKind, WindowOptions, actions,
+    image_cache, point, px, retain_all,
 };
 use image_viewer::ImageInfo;
 use migrate::{MigrationBanner, MigrationEvent, MigrationNotification, MigrationType};
@@ -1336,7 +1336,7 @@ fn show_markdown_app_notification<F>(
                 let primary_button_on_click = primary_button_on_click.clone();
                 cx.new(move |cx| {
                     MessageNotification::new_from_builder(cx, move |window, cx| {
-                        gpui::div()
+                        image_cache(retain_all("notification-cache"))
                             .text_xs()
                             .child(markdown_preview::markdown_renderer::render_parsed_markdown(
                                 &parsed_markdown.clone(),