Make GD's alpha consistent with other engines

GD expects alpha values to be 0-127, with 0 being opaque and 127 being
transparent (7-bit little-endian). Everything else uses 0 as
transparent and 255 as opaque (8-bit big-endian). This change converts
8-bit big-endian inputs to 7-bit little-endian when used by GD.

This change will be backwards incompatible, and may confuse people who
already know GD handles alpha this way, but it makes ngx_small_light
more consistent to use (and means users can swap engines more easily).