Replace DECLARE_ALIGNED_ with vpx_memalign()

DECLARE_ALIGNED (__declspec(align())) does not work as intended when
used on class data members:

Data in classes or structures is aligned within the class or structure
at the minimum of its natural alignment and the current packing setting
(from #pragma pack or the /Zp compiler option)

Change-Id: I304aaa6c3716fbfae24675ecf192f4b40787e83e