Removed presets, load all images via load directives (implements #9), removed obsolete code, added some improvements

examples/greyscale.gdshader

@@ -0,0 +1,10 @@
+shader_type canvas_item;
+
+//!load ./swamp.jpg
+
+void fragment() {
+	float b = (COLOR.r + COLOR.g + COLOR.b) / 3.0;
+	COLOR.r = b;
+	COLOR.g = b;
+	COLOR.b = b;
+}