If the client attaches a new SHM buffer with a different format from a
previous one then we ought to trigger a full upload so that GL can
allocate a new texture. Otherwise Weston would technically be doing
invalid operations because it would call glTexSubImage2D with a
different format from the one specified in glTexImage2D. Presumably it
would also mean GL would have to convert the buffer as it copies the
sub-region in which isn't ideal.

This patch makes it decide the GL format when the buffer is attached
instead of when processing the damage and it will set
needs_full_upload if it is different from what it used before.