Merge pull request #14740 from Greg15153/master

Added background to set background color in gm

gm/gm-tests.ts

@@ -78,6 +78,7 @@ gm(src)
 	.authenticate(password)
 	.autoOrient()
 	.backdrop()
+	.background(color)
 	.bitdepth(bits)
 	.blackThreshold(intensity)
 	.blackThreshold(r, g, b)

gm/index.d.ts

@@ -108,6 +108,7 @@ declare namespace m {
         authenticate(password: string): State;
         autoOrient(): State;
         backdrop(): State;
+        background(color: string): State;
         bitdepth(bits: number): State;
         blackThreshold(intensity: number): State;
         blackThreshold(red: number, green: number, blue: number, opacity?: number): State;