Fix delay between URL fetch and image download (#9452)

Fetch each source image URL immediately before downloading each image
instead of fetching all URLs and then downloading all images.

Source image URLs may change, so the downloader may fail if there is
too long a delay between fetching the image URL and downloading the
image.

(cherry picked from commit bbe0ab1dd0b51bcb91ff0a7f78555b078d73201e)

# Conflicts:
#	app/src/main/java/eu/kanade/tachiyomi/data/download/Downloader.kt

app/src/main/java/eu/kanade/tachiyomi/data/download/Downloader.kt

@@ -349,8 +349,13 @@ class Downloader(
 
             download.status = Download.State.DOWNLOADING
 
-            // Get all the URLs to the source images, fetch pages if necessary
+            // Start downloading images, consider we can have downloaded images already
-            pageList.filter { it.imageUrl.isNullOrEmpty() }.forEach { page ->
+            // Concurrently do 2 pages at a time
+            pageList.asFlow()
+                .flatMapMerge(concurrency = 2) { page ->
+                    flow {
+                        // Fetch image URL if necessary
+                        if (page.imageUrl.isNullOrEmpty()) {
                             page.status = Page.State.LOAD_PAGE
                             try {
                                 page.imageUrl = download.source.fetchImageUrl(page).awaitSingle()
@@ -359,11 +364,6 @@ class Downloader(
                             }
                         }
 
-            // Start downloading images, consider we can have downloaded images already
-            // Concurrently do 2 pages at a time
-            pageList.asFlow()
-                .flatMapMerge(concurrency = 2) { page ->
-                    flow {
                         withIOContext { getOrDownloadImage(page, download, tmpDir, dataSaver) }
                         emit(page)
                     }.flowOn(Dispatchers.IO)