added greyscale special effect

Assets/Scripts/Shaders/grayscale.gdshader

@@ -0,0 +1,11 @@
+shader_type canvas_item;
+
+uniform sampler2D screen_texture : hint_screen_texture, repeat_disable;
+uniform float saturation : hint_range(0.0, 1.0) = 1.0; // 0 = grayscale, 1 = full color
+
+void fragment() {
+	vec4 tex = texture(screen_texture, SCREEN_UV);
+	float gray = dot(tex.rgb, vec3(0.299, 0.587, 0.114)); // luminance weights
+	vec3 result = mix(vec3(gray), tex.rgb, saturation);
+	COLOR = vec4(result, tex.a);
+}