Move transparent background project setting to a subsection

This prevents the project setting from being located directly within
a root category, which is confusing from an UX perspective in the
project settings editor.

doc/classes/ProjectSettings.xml

@@ -585,7 +585,7 @@
 			[b]Note:[/b] This setting has no effect on the home indicator if [code]hide_home_indicator[/code] is [code]true[/code].
 		</member>
 		<member name="display/window/per_pixel_transparency/allowed" type="bool" setter="" getter="" default="false">
-			If [code]true[/code], allows per-pixel transparency for the window background. This affects performance, so leave it on [code]false[/code] unless you need it. See also [member display/window/size/transparent] and [member rendering/transparent_background].
+			If [code]true[/code], allows per-pixel transparency for the window background. This affects performance, so leave it on [code]false[/code] unless you need it. See also [member display/window/size/transparent] and [member rendering/viewport/transparent_background].
 		</member>
 		<member name="display/window/size/always_on_top" type="bool" setter="" getter="" default="false">
 			Forces the main window to be always on top.
@@ -610,7 +610,7 @@
 			[b]Note:[/b] This setting is ignored on iOS.
 		</member>
 		<member name="display/window/size/transparent" type="bool" setter="" getter="" default="false">
-			If [code]true[/code], enables a window manager hint that the main window background [i]can[/i] be transparent. This does not make the background actually transparent. For the background to be transparent, the root viewport must also be made transparent by enabling [member rendering/transparent_background].
+			If [code]true[/code], enables a window manager hint that the main window background [i]can[/i] be transparent. This does not make the background actually transparent. For the background to be transparent, the root viewport must also be made transparent by enabling [member rendering/viewport/transparent_background].
 			[b]Note:[/b] To use a transparent splash screen, set [member application/boot_splash/bg_color] to [code]Color(0, 0, 0, 0)[/code].
 			[b]Note:[/b] This setting has no effect if [member display/window/per_pixel_transparency/allowed] is set to [code]false[/code].
 		</member>
@@ -2256,7 +2256,7 @@
 		<member name="rendering/textures/webp_compression/lossless_compression_factor" type="float" setter="" getter="" default="25">
 			The default compression factor for lossless WebP. Decompression speed is mostly unaffected by the compression factor. Supported values are 0 to 100.
 		</member>
-		<member name="rendering/transparent_background" type="bool" setter="" getter="" default="false">
+		<member name="rendering/viewport/transparent_background" type="bool" setter="" getter="" default="false">
 			If [code]true[/code], enables [member Viewport.transparent_bg] on the root viewport. This allows per-pixel transparency to be effective after also enabling [member display/window/size/transparent] and [member display/window/per_pixel_transparency/allowed].
 		</member>
 		<member name="rendering/vrs/mode" type="int" setter="" getter="" default="0">

editor/plugins/node_3d_editor_plugin.cpp

@@ -2606,7 +2606,7 @@ void Node3DEditorViewport::_project_settings_changed() {
 	const bool use_taa = GLOBAL_GET("rendering/anti_aliasing/quality/use_taa");
 	viewport->set_use_taa(use_taa);
 
-	const bool transparent_background = GLOBAL_GET("rendering/transparent_background");
+	const bool transparent_background = GLOBAL_GET("rendering/viewport/transparent_background");
 	viewport->set_transparent_background(transparent_background);
 
 	const bool use_debanding = GLOBAL_GET("rendering/anti_aliasing/quality/use_debanding");

scene/main/scene_tree.cpp

@@ -1419,7 +1419,7 @@ SceneTree::SceneTree() {
 	ProjectSettings::get_singleton()->set_custom_property_info("rendering/anti_aliasing/quality/msaa_3d", PropertyInfo(Variant::INT, "rendering/anti_aliasing/quality/msaa_3d", PROPERTY_HINT_ENUM, String::utf8("Disabled (Fastest),2× (Average),4× (Slow),8× (Slowest)")));
 	root->set_msaa_3d(Viewport::MSAA(msaa_mode_3d));
 
-	const bool transparent_background = GLOBAL_DEF("rendering/transparent_background", false);
+	const bool transparent_background = GLOBAL_DEF("rendering/viewport/transparent_background", false);
 	root->set_transparent_background(transparent_background);
 
 	const int ssaa_mode = GLOBAL_DEF_BASIC("rendering/anti_aliasing/quality/screen_space_aa", 0);