avvideodec: ensure required mem alignment fixing avdec_h265 crashes with ximagesink...

avvideodec: ensure required mem alignment fixing avdec_h265 crashes with ximagesink/glimagesink

Make sure the alignment requirement in GstAllocationParams
matches the GstVideoAlignment requirements. This fixes
issues with avdec_h265 crashing in the avx2 code path when
used with playbin and ximagesink/glimagesink as videosink.

The internal video pool would allocate buffers with an
alignment of 15 even though GstVideoAlignment specified
a stride_align requirement of 31 (which comes from ffmpeg).

https://bugzilla.gnome.org/show_bug.cgi?id=754120