When capturing 1600x900, system could crash when system memory usage is
tight.

The way to reproduce this issue:
1. Use 1600x900 to display on host
2. Mount ISO through 'Virtual media' on OpenBMC's web
3. Run script as below on host to do sha continuously
  #!/bin/bash
  while [ [1] ];
  do
	find /media -type f -printf '"%h/%f"\n' | xargs sha256sum
  done
4. Open KVM on OpenBMC's web

The size of macro block captured is 8x8. Therefore, we should make sure
the height of src-buf is 8 aligned to fix this issue.
Signed-off-by: Jammy Huang <jammy_huang@aspeedtech.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>