Media: a shaped corner follows the pointer up and down; the fitted crop is told once

With an aspect, a corner drag sizes the crop by whichever side the pointer
moved further, so a drag straight up or down resizes it too. The upload
tells the host the fitted crop once per file, whether the host keeps it or
not; a host that dropped it had set off a loop of reports. The annotator's
Escape stops only when it lets a chosen mark go.

TASKS.md

@@ -309,3 +309,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T25f | 2 | PASS | |
 | T26a | 1 | FAIL | the viewer's picture had no id, so an animated picture never left its first frame |
 | T26a | 2 | PASS | |
+| T26b | 1 | FAIL | a shaped crop's corner ignored the pointer's move up or down |