GB Video: Fix enabling window when LY > WY (fixes #409)

CHANGES

@@ -32,6 +32,7 @@ Bugfixes:
  - GBA Timer: Fix timers sometimes being late (fixes mgba.io/i/1012)
  - GBA Hardware: Fix RTC overriding light sensor (fixes mgba.io/i/1069)
  - GBA Savedata: Fix savedata modified time updating when read-only
+ - GB Video: Fix enabling window when LY > WY (fixes mgba.io/i/409)
 Misc:
  - GBA Timer: Use global cycles for timers
  - GBA: Extend oddly-sized ROMs to full address space (fixes mgba.io/i/722)

src/gb/renderers/software.c

@@ -229,7 +229,11 @@ static void GBVideoSoftwareRendererUpdateWindow(struct GBVideoSoftwareRenderer*
 			renderer->hasWindow = true;
 		} else {
 			if (!renderer->hasWindow) {
-				renderer->currentWy = renderer->lastY - renderer->wy;
+				if (renderer->lastY > renderer->wy) {
+					renderer->currentWy = GB_VIDEO_VERTICAL_PIXELS;
+				} else {
+					renderer->currentWy = renderer->lastY - renderer->wy;
+				}
 			} else {
 				renderer->currentWy += renderer->lastY;
 			}