[idd] transport: abstract LGMP implementation

Introduce transport, frame, and control interfaces with an LGMP factory
backend.

Move LGMP and IVSHMEM implementation details under transport/lgmp and
expose direct frame-buffer memory through a neutral capability.

idd/LGIdd/transport/FrameMemoryLimits.h

@@ -24,8 +24,9 @@
 
 struct FrameMemoryLimits
 {
-  uint64_t sharedSize        = 0;
+  uint64_t capacity          = 0;
   uint64_t frameMemoryOffset = 0;
   uint64_t alignment         = 0;
   uint64_t maxFrameSize      = 0;
+  unsigned bufferCount       = 0;
 };