We used to only generate the favicon if it was specified in the user
project settings, now it's optional, will export it to `NAME.icon.png`,
(falling back to the default project icon if none is set in project
settings), and the `<link>` tag is added using the `$HEAD_INCLUDE`
instead of being hardcoded in the template.
Three canvas resize policies:
- `None`: Godot window settings are ignored.
- `Project`: Godot handles the canvas like a native app (resizing it
when setting the window size).
- `Adaptive`: Canvas size will always adapt to browser window size.
Use `None` if you want to control the canvas size with custom JavaScript
code.
This makes the project icon display immediately as a favicon when
opening the page, without having to wait for the project to finish
loading.
(cherry picked from commit 4492cf856b)
