Distinguish between 'add' and 'lighter'

When CAIRO_OPERATOR_ADD and CAIRO_OPERATOR_LIGHTEN are both available,
make CAIRO_OPERATOR_ADD available as 'add'.
Giflib 5.1

update to compile with giflib >= 5.1
Update Image.cc
Update Image.h
Giflib 5.1
Glib version test into giflib exists test

Include test of glib version into the have gif lib test
export canvas.CanvasPixelArray instead of canvas.PixelArray which is undefined
Merge pull request #447 from svandecappelle/master

giflib 5.1
Merge pull request #448 from Gottox/master

export canvas.CanvasPixelArray instead of canvas.PixelArray which is undefined
Merge pull request #381 from mojodna/add-operator

Distinguish between 'add' and 'lighter'
History: consistent output styling
Release 1.1.6