gg: revamp the Context pipeline for more effects, implement an additive effect (#16394)

vlib/gg/image.v

@@ -16,7 +16,14 @@ pub:
 	part_rect Rect // defines the size and position of part of the image to use when rendering
 	rotate    int  // amount to rotate the image in degrees
 	z         f32
-	color     gx.Color = gx.white
+	color     gx.Color    = gx.white
+	effect    ImageEffect = .alpha
+}
+
+pub enum ImageEffect {
+	// TODO(FireRedz): Add more effects
+	alpha
+	add
 }
 
 // Rect represents a rectangular shape in `gg`.