buffer: track asynchronous buffers and don't release them until unref

synchronous buffers are read instantly and we can release them, but asynchronous ones have to be locked until they are unref'd from .current to avoid reading from a buffer after .release()

src/protocols/core/Shm.hpp

@@ -37,6 +37,7 @@ class CWLSHMBuffer : public IWLBuffer {
     virtual eBufferCapability                      caps();
     virtual eBufferType                            type();
     virtual void                                   update(const CRegion& damage);
+    virtual bool                                   isSynchronous();
     virtual SSHMAttrs                              shm();
     virtual std::tuple<uint8_t*, uint32_t, size_t> beginDataPtr(uint32_t flags);
     virtual void                                   endDataPtr();