[client] transport: introduce pluggable frame transports

Move shared-memory ownership, LGMP session handling, queue access, and
DMA setup behind a transport interface. The LGMP backend preserves the
existing zero-copy frame and DMA paths while owning its lgmp:* options.

Expose the initialized EGL context through a versioned renderer interop
record for future accelerated decode backends. Add an LGMP-independent,
deterministic test transport for graphics-pipeline validation.

client/src/transport.c

@@ -0,0 +1,59 @@
+/**
+ * Looking Glass
+ * Copyright © 2017-2026 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#include "interface/transport.h"
+#include "dynamic/transports.h"
+
+#include <string.h>
+
+void lgTransport_setup(void)
+{
+  for (unsigned i = 0; i < LG_TRANSPORT_COUNT; ++i)
+    if (LG_Transports[i]->setup)
+      LG_Transports[i]->setup();
+}
+
+bool lgTransport_isValid(const char * name)
+{
+  if (!name)
+    return false;
+  for (unsigned i = 0; i < LG_TRANSPORT_COUNT; ++i)
+    if (strcmp(LG_Transports[i]->name, name) == 0)
+      return true;
+  return false;
+}
+
+bool lgTransport_create(const char * name, LG_Transport ** transport,
+    const LG_TransportOps ** ops)
+{
+  for (unsigned i = 0; i < LG_TRANSPORT_COUNT; ++i)
+  {
+    if (strcmp(LG_Transports[i]->name, name) != 0)
+      continue;
+
+    if (!LG_Transports[i]->create(transport))
+      return false;
+
+    *ops = LG_Transports[i];
+    return true;
+  }
+
+  return false;
+}