st/va: add colospace conversion through Video Post Processing

st/va: add colospace conversion through Video Post Processing

Add support for VPP in the following functions:
vlVaCreateContext
vlVaDestroyContext
vlVaBeginPicture
vlVaRenderPicture
vlVaEndPicture

Add support for VAProcFilterNone in:
vlVaQueryVideoProcFilters
vlVaQueryVideoProcFilterCaps
vlVaQueryVideoProcPipelineCaps

Add handleVAProcPipelineParameterBufferType helper.

One application is:
VASurfaceNV12 -> gstvaapipostproc -> VASurfaceRGBA

Signed-off-by: Julien Isorce <j.isorce@samsung.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>