buffer: add a NONE flag to eBufferCapability (#11)

ensure we dont cast out of range in caps(),
return (Aquamarine::eBufferCapability)0; in GBM.cpp

include/aquamarine/buffer/Buffer.hpp

@@ -8,6 +8,7 @@
 
 namespace Aquamarine {
     enum eBufferCapability : uint32_t {
+        BUFFER_CAPABILITY_NONE    = 0,
         BUFFER_CAPABILITY_DATAPTR = (1 << 0),
     };