GPU/HW: Disable depth buffering on transparent polygons

These are the most likely to be coplanar and cause issues.

src/core/gpu_hw.cpp

@@ -576,7 +576,9 @@ void GPU_HW::LoadVertices()
         }
         else if (g_settings.gpu_pgxp_depth_buffer)
         {
-          SetBatchDepthBuffer(true);
+          const bool use_depth = (m_batch.transparency_mode == GPUTransparencyMode::Disabled);
+          SetBatchDepthBuffer(use_depth);
+          if (use_depth)
             CheckForDepthClear(vertices.data(), num_vertices);
         }
       }