[MG_Backend] (Magma): reconcile a shader-written element buffer before the draw records

The client-memory upload's index scan waits for the GPU (SyncGpuWrites), and that
wait flushes the batch the frame's command buffer belongs to - so a handle taken
before it is stale by the time the draw records into it. Reconciling in the entry
point, before SetupDraw starts recording, keeps the wait out of the recording.

MobileGL/MG_Backend/DirectVulkan/DirectVulkan.cpp

@@ -1045,6 +1045,30 @@ namespace MobileGL::MG_Backend::DirectVulkan {
         payload.params.indexCount = count;
         payload.params.instanceCount = 1;
 
+        // A CLIENT-MEMORY VERTEX ARRAY'S upload is bounded by a scan of this draw's index bytes,
+        // and those bytes may be shader-written: the scan reconciles the element buffer's shadow,
+        // which WAITS for the GPU. A wait in the middle of the draw's own recording flushes the
+        // batch that command buffer belongs to - the handle SetupDraw is holding goes stale, and
+        // the next vkCmd* records into a command buffer that is no longer the frame's. Reconciling
+        // HERE, before SetupDraw starts recording, keeps the wait out of the recording.
+        if (MG_Config::Transport == MG_Config::TransportMode::Monolith) {
+            const auto& currentVAO = MGB_CTX->GetBoundVertexArray();
+            Bool clientArray = false;
+            if (currentVAO) {
+                for (const auto& attribute : currentVAO->GetAllAttributes()) {
+                    if (attribute.Enabled && !attribute.Buffer) {
+                        clientArray = true;
+                        break;
+                    }
+                }
+            }
+            if (clientArray) {
+                if (const auto& elementBuffer = currentVAO->GetIndexBufferBindingSlot().GetBoundObject()) {
+                    elementBuffer->SyncGpuWrites();
+                }
+            }
+        }
+
         pVulkanRenderer->DrawElements(payload);
     }