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 05e2849087)

(cherry picked from commit 91bdaa6c87)

Otherwise we might be tricked in to reading and writing things at
incorrect offsets for pixels which ultimately could result in an
attacker writing things to the stack or heap and executing things
they shouldn't.

This only affects the server as the client never uses the pixel
format suggested by th server.

Issue found by Pavel Cheremushkin from Kaspersky Lab.

(cherry picked from commit cd1d650c53)

(cherry picked from commit f1b9b868ec)

(cherry picked from commit 014c501237)

They have very different purpose, so make things easier to work
with by having multiple directories.