Automatic merge of T1.5.1-905-gaf8fb30c5 and 15 pull requests

- Pull request #570 at 3539862: Experimental glTF 2.0 support with PBR lighting
- Pull request #839 at d00beb9: First phase of https://blueprints.launchpad.net/or/+spec/additional-cruise-control-parameters
- Pull request #876 at f92de76: docs: add source for documents previously on website to source Documentation folder
- Pull request #882 at 9c456aa: Blueprint/train car operations UI window
- Pull request #885 at 8f94333: feat: Add notifications to Menu
- Pull request #886 at 6c0785b: Scene viewer extension to TrackViewer
- Pull request #892 at 1f5ba4c: Signal Function OPP_SIG_ID_TRAINPATH
- Pull request #896 at 5866028: First implementation of https://blueprints.launchpad.net/or/+spec/specific-sounds-for-ai-trains
- Pull request #903 at 9bead33: Downloading route content (Github, zip)
- Pull request #911 at 6834af0: docs: Add refactoring as a special type of PR
- Pull request #916 at 992cc97: Distributed Power Air Brake Synchronization
- Pull request #919 at 9358732: Added mouse wheel support for controls which can be moved by pressing t…
- Pull request #920 at a94e403: Update RailDriver in Manual
- Pull request #923 at d0208cd: Add curve squeal to route
- Pull request #924 at 6c2c3cd: Default Asset Improvements

Author: openrails-bot

Source/Documentation/Manual/news.rst

@@ -33,6 +33,7 @@ What's been improved
 
 - 64-bit compatibility lifting the 3GB RAM limit
 - Better use of VRAM
+- New default skybox, light, and snowflake assets contributed by community members. Thanks to Erick Cantu, Paul Peterson, and BaldoBi for their contributions!