DISPCNT enable bits for BG3 in Mode 1, BG0/BG1 in Mode 2, and every background
but BG2 in Modes 3-5 failed the whole frame with UnsupportedLayers, and the
window turned that into a stopped session. Hardware has no such backgrounds and
ignores the bits, so the renderer now masks them. The error variant is removed.
Prohibited OBJ mode 3 and shape 3 stay diagnostics: their hardware output is
not established here.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhfHfHEuzv2AUTd6erW1gU
0ef2719c6b