Add a Viewport property to use full floating-point precision in HDR

This is only available on the GLES3 backend.

This can be useful for advanced shaders, but it should generally
not be enabled otherwise as full precision has a performance cost.
For general-purpose rendering, the built-in debanding filter should
be used to reduce banding instead.

servers/visual/rasterizer.h

@@ -562,6 +562,7 @@ public:
 		RENDER_TARGET_HDR,
 		RENDER_TARGET_KEEP_3D_LINEAR,
 		RENDER_TARGET_DIRECT_TO_SCREEN,
+		RENDER_TARGET_USE_32_BPC_DEPTH,
 		RENDER_TARGET_FLAG_MAX
 	};