Add unlit rendering for M2 glow/additive batches

Batches with the M2 unlit material flag (0x01) or additive blend modes
(3+) now skip lighting, shadows, and fog, emitting texture color directly.
Fixes lantern glow quads appearing as dull transparent circles.

include/rendering/m2_renderer.hpp

@@ -33,6 +33,7 @@ struct M2ModelGPU {
         bool hasAlpha = false;
         uint16_t textureAnimIndex = 0xFFFF; // 0xFFFF = no texture animation
         uint16_t blendMode = 0;   // 0=Opaque, 1=AlphaKey, 2=Alpha, 3=Add, etc.
+        uint16_t materialFlags = 0; // M2 material flags (0x01=Unlit, 0x04=TwoSided, 0x10=NoDepthWrite)
     };
 
     GLuint vao = 0;