Compute alignment correctly

Compute alignment correctly

The code previously assumed that everything has to be aligned to a 4 byte
boundary. This assumption is wrong as e.g. the STR struct from xproto shows.

Instead, each type has to be aligned to its natural alignment. So a char doesn't
need any alignment, a INT16 gets aligned to a 2-byte-boundary and a INT32 gets
the old 4 byte alignment.

I'm not 100% sure that this commit is correct, but some quick tests with awesome
and cairo-xcb went well.

This commit causes lots of dead assignments to xcb_align_to since only the last
field's alignment is actually used, but this simplified this patch a lot.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=34037

Signed-off-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Peter Harris <pharris@opentext.com>