Ensure MovieWriter output is in gamma space when using HDR 2D

doc/classes/ViewportTexture.xml

@@ -8,6 +8,11 @@
 		To get a [ViewportTexture] in code, use the [method Viewport.get_texture] method on the target viewport.
 		[b]Note:[/b] A [ViewportTexture] is always local to its scene (see [member Resource.resource_local_to_scene]). If the scene root is not ready, it may return incorrect data (see [signal Node.ready]).
 		[b]Note:[/b] Instantiating scenes containing a high-resolution [ViewportTexture] may cause noticeable stutter.
+		[b]Note:[/b] When using a [Viewport] with [member Viewport.use_hdr_2d] set to [code]true[/code] the returned texture will be an HDR image encoded in linear space. This may look darker than normal when displayed directly on screen. To convert to gamma space, you can do the following:
+		[codeblock]
+		img.convert(Image.FORMAT_RGBA8)
+		imb.linear_to_srgb()
+		[/codeblock]
 	</description>
 	<tutorials>
 		<link title="GUI in 3D Viewport Demo">https://godotengine.org/asset-library/asset/2807</link>