Fix unaligned XFB stores in GL44-CTS.vertex_attrib_64bit.vao test

Fix unaligned XFB stores in GL44-CTS.vertex_attrib_64bit.vao test

This change aligns  result buffer passed through transform
feedback to 8 (required for storing fp64 vaules).

Spec:
"The results of appending a
    varying variable to a transform feedback buffer are undefined if any
    component of that variable would be written at an offset not aligned to
    the size of the component."

Components: OpenGL
VK-GL-CTS issue: 264

Affects test:
GL44-CTS.vertex_attrib_64bit.vao

Change-Id: I81f7b0e42a8ef0a0c282d63d255fbed265e81c7b