doc/go1.2.txt: document new functions/types added to image/gif and image/draw.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/11492045

doc/go1.2.txt

@@ -21,6 +21,9 @@ encoding/json: accept but correct invalid UTF-8 in Marshal (CL 11211045).
 flag: add Getter interface (CL 10472043).
 fmt: indexed access to arguments in Printf etc. (CL 9680043).
 go/build: support including C++ code with cgo (CL 8248043).
+image/draw: added Drawer, FloydSteinberg and the op.Draw method (CL 10977043).
+image/draw: added Quantizer type (CL 11148043).
+image/gif: added Encode and EncodeAll (CL 10896043).
 io: Copy prioritizes WriterTo over ReaderFrom (CL 9462044).
 net: new build tag netgo for building a pure Go net package (CL 7100050).
 sort: new Stable function provides stable sort (CL 9612044).