fix(vips): fix fill(none) for grayscale image (#422)

* fix(vips): fix fill(none) for grayscale image

* test: update golden files

Author: cshum

testdata/golden/fit-in/100x100/filters%3Afill%28none%29/2bands.png

@@ -215,6 +215,11 @@ func (v *Processor) fill(ctx context.Context, img *Image, w, h int, pLeft, pTop,
 			}
 		}
 		if isTransparent {
+			if img.Bands() < 3 {
+				if err = img.ToColorSpace(InterpretationSRGB); err != nil {
+					return
+				}
+			}
 			if err = img.AddAlpha(); err != nil {
 				return
 			}

vips/filter.go

@@ -125,6 +125,7 @@ func TestProcessor(t *testing.T) {
 			{name: "crop-percent stretch top flip", path: "0.006120x0.008993:1.0x1.0/stretch/100x200/filters:brightness(-20):contrast(50):rgb(10,-50,30):fill(black)/gopher.png"},
 			{name: "padding rotation fill blur grayscale", path: "/fit-in/200x210/20x20/filters:rotate(90):rotate(270):rotate(180):fill(blur):grayscale()/gopher.png"},
 			{name: "fill round_corner", path: "fit-in/0x210/filters:fill(yellow):round_corner(40,60,green)/gopher.png"},
+			{name: "grayscale fill none", path: "fit-in/100x100/filters:fill(none)/2bands.png"},
 			{name: "trim alpha", path: "trim/find_trim_alpha.png"},
 			{name: "trim with crop", path: "trim:bottom-right/50x50:0x0/find_trim.png"},
 			{name: "trim right", path: "trim:bottom-right/500x500/filters:strip_exif():upscale():no_upscale()/find_trim.png"},