FEAT: possibility to count average image color using /color

For example having black&white image:
```
i: make image! 2x1 i/1: 0.0.0
```
One can count average color using:
```
i/color ;== 127.127.127.255
```

Using `/color:` will set RGB components of all pixels (ignoring alpha channel)

Author: Oldes

src/core/t-image.c

@@ -411,6 +411,39 @@
 	}
 }
 
+/***********************************************************************
+**
+*/	void Average_Image_Color(REBYTE *rgba, REBVAL *clr, REBINT len)
+/*
+***********************************************************************/
+{
+	REBU64 r = 0, g = 0, b = 0, a = 0;
+	REBU64 n = (REBU64)len;
+
+	VAL_SET(clr, REB_TUPLE);
+	VAL_TUPLE_LEN(clr) = 4;
+	if (n > 0) {
+		for (; len > 0; len--, rgba += 4) {
+			r += rgba[C_R];
+			g += rgba[C_G];
+			b += rgba[C_B];
+			a += rgba[C_A];
+		}
+		VAL_TUPLE(clr)[0] = (REBYTE)(r / n);
+		VAL_TUPLE(clr)[1] = (REBYTE)(g / n);
+		VAL_TUPLE(clr)[2] = (REBYTE)(b / n);
+		VAL_TUPLE(clr)[3] = (REBYTE)(a / n);
+	}
+	else {
+		// returning transparent black when image is empty
+		VAL_TUPLE(clr)[0] = 0;
+		VAL_TUPLE(clr)[1] = 0;
+		VAL_TUPLE(clr)[2] = 0;
+		VAL_TUPLE(clr)[3] = 0;
+		//.. or should it return none instead?
+	}
+}
+
 
 #ifdef ndef
 INLINE REBCNT ARGB_To_BGR(REBCNT i)
@@ -1330,6 +1363,10 @@ INLINE REBCNT ARGB_To_BGR(REBCNT i)
 				Set_Binary(val, nser);
 				break;
 
+			case SYM_COLOR:
+				Average_Image_Color(src, val, len);
+				break;
+
 			default:
 				return PE_BAD_SELECT;
 			}
@@ -1346,6 +1383,9 @@ INLINE REBCNT ARGB_To_BGR(REBCNT i)
 				break;
 
 			case SYM_RGB:
+			case SYM_COLOR:
+				// SYM_COLOR as a setter is used to count average color of the image,
+				// here we can will all pixels with it for consistency
 				if (IS_TUPLE(val)) {
 					Fill_Line((REBCNT *)src, TO_PIXEL_TUPLE(val), len, 1);
 				} else if (IS_INTEGER(val)) {

src/tests/units/image-test.r3

@@ -111,6 +111,29 @@ Rebol [
 ===end-group===
 
 
+===start-group=== "image/color"
+
+	--test-- "image/color getter"
+		;-- counts average image color
+		img: make image! 2x1
+		--assert img/color = 255.255.255.255 ; default image color is white
+		img/1: 0.0.0
+		--assert img/color = 127.127.127.255
+		img/2: 50.130.60.200
+		--assert img/color = 25.65.30.227
+
+	--test-- "image/color setter"
+		;-- just for consistency, fills image as if /rgb would be used
+		;-- alpha channel is ignored!
+		; using image state from previous test..
+		--assert img/rgba = #{000000FF32823CC8}
+		img/color: 25.65.30.227
+		--assert img/1 = 25.65.30.255
+		--assert img/2 = 25.65.30.200
+
+===end-group===
+
+
 ===start-group=== "image pixel assignment validity"
 	img: make image! 2x2
 	--test-- "image pixel 3-tuple assignment"