GPencil: Invert Fade Object slider

The Fade Objects and Fade Layers parameters had opposite meaning and this was not correct.

Now, both parameters work in the same direction.

source/blender/draw/engines/gpencil/shaders/gpencil_paper_frag.glsl

@@ -5,5 +5,5 @@ out vec4 FragColor;
 
 void main()
 {
-  FragColor = vec4(color, opacity);
+  FragColor = vec4(color, 1.0 - opacity);
 }