Make colors of ML recording viewport more similar to VR

The VR mirror view has to keep the data linear, as OpenVR expects linear
data and converts to sRGB itself. This won't be fixed in the near future
(see the godot_openvr project readme for details), so for the ML
training data, we also use linear colors.

Imaging/Dolly/Dolly.tscn

@@ -33,6 +33,7 @@ __meta__ = {
 size = Vector2( 630, 430 )
 handle_input_locally = false
 hdr = false
+keep_3d_linear = true
 render_target_update_mode = 0
 shadow_atlas_size = 4
 script = ExtResource( 4 )