Add a debanding property to Viewport

It can be enabled in the Project Settings
(`rendering/quality/screen_filters/use_debanding`). It's disabled
by default as it has a small performance impact and can make
PNG screenshots much larger (due to how dithering works).
It will also slightly brighten the scene's dark areas.

As a result, it should be enabled only when banding is noticeable enough.

This closes #17006.

servers/rendering/rasterizer_rd/rasterizer_effects_rd.h

@@ -184,7 +184,7 @@ class RasterizerEffectsRD {
 
 		float pixel_size[2];
 		uint32_t use_fxaa;
-		uint32_t pad;
+		uint32_t use_debanding;
 	};
 
 	/* tonemap actually writes to a framebuffer, which is
@@ -628,6 +628,7 @@ public:
 		RID color_correction_texture;
 
 		bool use_fxaa = false;
+		bool use_debanding = false;
 		Vector2i texture_size;
 	};