Unknown picture structure

[xhaihao]

The picture structure in the output parameters returned from
MFXVideoDECODE_Query() is set to MFX_PICSTRUCT_UNKNOWN for some codecs.
The structure of the corresponding mfx surfaces created for decoding is
unknown if we pass the returned parameters from MFXVideoDECODE_Query() to MFXVideoDECODE_Init(), a pipeline using these mfx surfaces as the
input surfaces for next video process will be broken by unknown picture structure.

Example command line to reproduce the issue:

gst-launch-1.0 filesrc location=input_vp8.webm ! matroskademux !
msdkvp8dec ! msdkvpp ! fakesink