[Xcb] [PATCH] Force XCB event structures with 64-bit extended fields to be packed.

[Mark Kettenis]

>  With the advent of the Present extension, some events (such as
>  PresentCompleteNotify) now use native 64-bit types on the wire.
>
>  For XGE events, we insert an extra "uint32_t full_sequence" field
>  immediately after the first 32 bytes of data.  Normally, this causes
>  the subsequent fields to be shifted over by 4 bytes, and the structure
>  to grow in size by 4 bytes.  Everything works fine.
>
>  However, if event contains 64-bit extended fields, this may result in
>  the compiler adding an extra 4 bytes of padding so that those fields
>  remain aligned on 64-bit boundaries.  This causes the structure to grow
>  by 8 bytes, not 4.  Unfortunately, XCB doesn't realize this, and
>  always believes that the length only increased by 4.  read_packet()
>  then fails to malloc enough memory to hold the event, and the event
>  processing code uses the wrong offsets.
>
>  To fix this, mark any event structures containing 64-bit extended
>  fields with __attribute__((__packed__)).

The problem with that approach is that this is a GCC-ism that isn't
portable.  I believe the sun studio compiler supports something similar. 
But other compilers might not.

C11 has _Alignas(), but it is a tad bit early to start relying on this.

I also don't quite trust the compiler to get to emit the right
instructions for unaligned access on all platforms.