group gid and group name, in libgamma-error, are thread local

Signed-off-by: Mattias Andrée <maandree@operamail.com>
make figure_out_card_open_error conditionally thread-safe

Signed-off-by: Mattias Andrée <maandree@operamail.com>
test other bit-depths, 8 bits is not working when 16 bits is native

Signed-off-by: Mattias Andrée <maandree@operamail.com>
update gamma-helper to support 8-bit ramps

Signed-off-by: Mattias Andrée <maandree@operamail.com>