I've just discovered iHD driver in Skylake doesn't have VideoProc
entry point, hence, in this platform, when vaapioverlay is tried to be
registered, critical warnings are raised because blend doesn't have a
display assigned.
As it is possible to have drivers without EntryPointVideoProc it is
required to handle it gracefully. This patch does that: only tries to
register vaapioverlay if the testing display has VPP and finalize()
vmethods, in filter and blend, bail out if display is NULL.
Instead of using a parent structure that has to be derived by API
consumers, this change propse a simplification by using the common
pattern of GTK of passing a function pointer and user data which will
be passed as its parameter. That user data contains the state and the
function will be called to update that state.
This new API allows the user to call a single method (process)
which handles the [display] lock/unlock logic internally for
them.
This API supersedes the risky begin, render, end API.
It eliminates the need for the user to call a lock method
(process_begin) before processing the input buffers
(process_render) and calling an unlock method (process_end)
afterwards.
See #219
