Parse M2 render flags and apply per-batch blend modes

Water/lava batches in fountain and Ironforge M2 models use non-opaque
blend modes (alpha, additive) defined in the M2 material table. Without
parsing these, they rendered as solid surfaces extending visibly beyond
their containers. Now each batch looks up its blend mode from the
material array and sets the appropriate GL blend function.

include/rendering/m2_renderer.hpp

@@ -32,6 +32,7 @@ struct M2ModelGPU {
         uint32_t indexCount = 0;
         bool hasAlpha = false;
         uint16_t textureAnimIndex = 0xFFFF; // 0xFFFF = no texture animation
+        uint16_t blendMode = 0;   // 0=Opaque, 1=AlphaKey, 2=Alpha, 3=Add, etc.
     };
 
     GLuint vao = 0;