Add a debanding property to Viewport for GLES3

It can be enabled in the Project Settings
(`rendering/quality/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).

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

Since debanding requires a HDR viewport to work, it's only supported
in the GLES3 backend.

servers/visual_server.h

@@ -681,6 +681,7 @@ public:
 
 	virtual void viewport_set_msaa(RID p_viewport, ViewportMSAA p_msaa) = 0;
 	virtual void viewport_set_use_fxaa(RID p_viewport, bool p_fxaa) = 0;
+	virtual void viewport_set_use_debanding(RID p_viewport, bool p_debanding) = 0;
 
 	enum ViewportUsage {
 		VIEWPORT_USAGE_2D,