message: Use the fast approximation on mobile instead of desktop
diff: #if defined(TARGET_MOBILE)
  #define BRDF_SPECULAR_D SPECULAR_D_GGX
  -#define BRDF_SPECULAR_V SPECULAR_V_SMITH_GGX
  +#define BRDF_SPECULAR_V SPECULAR_V_SMITH_GGX_FAST
  #define BRDF_SPECULAR_F SPECULAR_F_SCHLICK
  #else
  #define BRDF_SPECULAR_D SPECULAR_D_GGX
  -#define BRDF_SPECULAR_V SPECULAR_V_SMITH_GGX_FAST
  +#define BRDF_SPECULAR_V SPECULAR_V_SMITH_GGX
  #define BRDF_SPECULAR_F SPECULAR_F_SCHLICK
  #endif
  