d3d11decoder: Ensure the written bitstream buffer size is 128 bytes aligned

DXVA spec is saying that the size of bitstream buffer provided by hardware decoder should be 128 bytes aligned. And also the host software decoder should align the size of written buffer to 128 bytes. That means if the slice (or frame in case of VP9) size is not aligned with 128 bytes, the rest of non 128 bytes aligned memory should be zero-padded.

In addition to aligning implementation, some variables are renamed to be more intuitive by this commit.