Make GD's alpha consistent with other engines

[SteveMarshall]

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).

This fixes #9.

[cubicdaiya]

Hi, @SteveMarshall

Tnanks for submmitting your patch.

Though I checked your patch and recognized GD's alpha channel problem,
But It looks not good to me after applying your patch except alpha-value is 0x00 or 0xff.
For example, e=gd is not consistent with other engines when alpha-value is 0xab.

And I don't know how to convert an alpha-value which range is 0255 to which alpha-value which range is 0127 and inverted correctly.

Do you have any idea?

[SteveMarshall]

I think the inconsistency between engines is actually because the other engines aren't using the alpha channel properly. When I use cc=ff000000 with ImageMagick, it shows the canvas as red instead of transparent, black, or white (I’d expect one of those).

With e=gd and my patch, cc=ff000000 shows black (GD's default background colour), and with cc=ff0000ab, it shows red blended with black, which I think is the correct behaviour.