HUD: stretch full-screen layers of moved windows across the render width

Primitives covering the whole 1024x768 layout inside a moved window, such as
the dimming behind the exit confirmation, were shifted with the window and
left both sides undimmed; they are stretched to the render width instead.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/hud.md

@@ -52,3 +52,9 @@ sample a position ~400 px off now and then (jerky hold-to-move). Every user of
 the cursor goes through the accessor `0x6550f0`, so inside a moved window it
 returns a thread-local copy instead (with its own CRITICAL_SECTION at +0x24;
 copying and writing back hold the real object's lock).
+
+## Full-screen layers
+
+A primitive drawn inside a moved window that covers the whole 1024x768 layout
+(e.g. the dimming behind the exit confirmation) is stretched across the render
+width instead of shifted.