Rework of #56959

Its been a few years since that PR has been modified (originally made for 4.0) so I started again with 4.5, solved any conflicts with the previous one, and made some additions with how new platform specific splash screens are handled.

To rehash, this will replace the application/boot_splash/fullsize project setting that can only be toggled on or off in favor of various stretching options that change how the boot image will appear for Godot projects on startup.

It's a full upgrade because the stretching options include what toggling full size on or off meant previously and with the new options. However, this mostly applies to the editor boot splash and exported desktop platforms because mobile platforms have their own platform-specific approaches for showing boot splashes.

The new setting is called application/boot_splash/stretch_mode and the options include:

• Disabled (same as toggling the legacy full size project setting off, no stretching applied)
• Keep (same as toggling the legacy full size project setting on, stretches while preserving aspect ratio)
• Keep Width (stretches width based on height)
• Keep Height (stretches height based on the width)
• Cover (stretches until it covers the entire screen while preserving aspect ratio)
• Expand (stretches until it covers the entire screen but doesn't preserve aspect ratio)

To preserve compatibility, set_boot_splash retains the same signature but now routes to a new function set_boot_splash_with_stretch which is what's now used for the various renderers. Also, for projects that still have the legacy project setting serialized, a check is done to convert that to the corresponding stretch option.