LibSoftGPU: Be less lenient towards unknown enum values

Userland/Libraries/LibSoftGPU/Device.cpp

@@ -707,7 +707,6 @@ void Device::submit_triangle(const Triangle& triangle, Vector<size_t> const& ena
  // FIXME: Implement more blend modes
  switch (sampler.config().fixed_function_texture_env_mode) {
  case TextureEnvMode::Modulate:
- default:
  fragment = fragment * texel;
  break;
  case TextureEnvMode::Replace:
@@ -721,6 +720,8 @@ void Device::submit_triangle(const Triangle& triangle, Vector<size_t> const& ena
  fragment.set_z(texel.z() * src_alpha + fragment.z() * one_minus_src_alpha);
  break;
  }
+ default:
+ VERIFY_NOT_REACHED();
  }
  }
 
@@ -739,7 +740,7 @@ void Device::submit_triangle(const Triangle& triangle, Vector<size_t> const& ena
  factor = exp(-((m_options.fog_density * z) * (m_options.fog_density * z)));
  break;
  default:
- break;
+ VERIFY_NOT_REACHED();
  }
 
  // Mix texel's RGB with fog's RBG - leave alpha alone