[Fix] (DirectVulkan): flag a transform feedback capture as a GPU write

A capture is a GPU write like any shader's, so a later CPU read of the buffer has
to wait for it. Only shader storage buffers were flagged, so mapping or reading
back a capture buffer could observe whatever the queue had retired so far.

Nothing needs copying -- the capture writes land in coherent host-visible storage
already -- but coherence only says the writes are visible once they have
happened, which is exactly what MarkGpuWritten arranges through the readback op.

MobileGL/MG_Backend/DirectVulkan/Renderer/VulkanRenderer.cpp

@@ -8086,8 +8086,11 @@ void main() {
                 return false;
             }
             // Host-visible coherent GPU residency: the capture writes land where
-            // MapBuffer/GetBufferSubData read.
+            // MapBuffer/GetBufferSubData read. Coherence makes them visible once they
+            // have happened, so the buffer is also flagged for the wait that a later CPU
+            // read has to perform - the capture is a GPU write like any shader's.
             bufferObject->EnsureGpuResidentStorage();
+            bufferObject->MarkGpuWritten();
             BufferSlice slice{};
             if (!m_bufferManager.AcquireResidentSlice(BufferKind::Vertex, bufferObject, slice)) {
                 MGLOG_E("BeginXfbCaptureForDraw: failed to acquire capture buffer %zu", i);