Add a cache for gallery thumbnails (#496)

pkg/models/model_gallery.go

@@ -26,6 +26,8 @@ type Gallery struct {
 	UpdatedAt SQLiteTimestamp `db:"updated_at" json:"updated_at"`
 }
 
+const DefaultGthumbWidth int = 200
+
 func (g *Gallery) GetFiles(baseURL string) []*GalleryFilesType {
 	var galleryFiles []*GalleryFilesType
 	filteredFiles, readCloser, err := g.listZipContents()
@@ -152,3 +154,11 @@ func reorder(a []*zip.File, toFirst int) []*zip.File {
 	}
 	return a
 }
+
+func (g *Gallery) ImageCount() int {
+	images, _, _ := g.listZipContents()
+	if images == nil {
+		return 0
+	}
+	return len(images)
+}