Handle pixel formats with odd shift values

Handle pixel formats with odd shift values

Our fast paths assume that each channel fits in to a separate byte.
That means the shift needs to be a multiple of 8. Start actually
checking this so that a client cannot trip us up and possibly cause
incorrect code exection.

Issue found by Pavel Cheremushkin from Kaspersky Lab.

(cherry picked from commit 05e28490873a861379c943bf616614b78b558b89)