kmsalphablending: Replace KmsGenericStructure for KmsRefStruct
kmsalphablending: Replace KmsGenericStructure for KmsRefStruct

KmsRefStruct is a lighther structure wich is expected to be enough
for cases where you just need to provide data among callbacks keeping
references among them and release the data when the last ref reaches
the value of zero.

Change-Id: I5bd555bc7de1637efeebaaa2ce3a768ff2bf7235

TribeMedia/kms-elements,shelsonjava/kms-elements,shelsonjava/kms-elements,TribeMedia/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements