Fix image leaving behind a black rectangle when switching away and back to alternate screen

layer_dirty was being reset if remove_images was called more than once
without an intervening call to update_layers.

Fixes #2901

docs/changelog.rst

@@ -16,6 +16,9 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
 
 - Adjust cell height automatically for buggy fonts that draw brackets outside the bounding box
 
+- Fix image leaving behind a black rectangle when switch away and back to
+  alternate screen (:iss:`2901`)
+
 
 0.18.2 [2020-07-28]
 --------------------

kitty/graphics.c

@@ -646,7 +646,7 @@ grman_update_layers(GraphicsManager *self, unsigned int scrolled_by, float scree
 
 static inline void
 filter_refs(GraphicsManager *self, const void* data, bool free_images, bool (*filter_func)(ImageRef*, Image*, const void*, CellPixelSize), CellPixelSize cell) {
-    self->layers_dirty = self->image_count > 0;
+    if (self->image_count) self->layers_dirty = true;
     for (size_t i = self->image_count; i-- > 0;) {
         Image *img = self->images + i;
         for (size_t j = img->refcnt; j-- > 0;) {