Only 64k are allowed for the sum of all private instance structs in the
class hierarchy, as well as for the public instance structs.
The CdgInterpreter itself is huge and adding just another two integers
to GstVideoDecoderPrivate in libgstvideo is causing the limit to be
reached, so let's allocate it in a separate memory area.
Test was relying on 'blocksize' property on the source which can
only be used in push mode.
This change in baseparse broke it:
e906197c62
so ensure we are actually in push mode by using pushfilesrc.
