Viewport 2D in 3D material cleanups

[Malcolmnixon]

This pull request implements feature request #488 by:

• Allowing the user to specify a custom material for the screen
• Hide material overrides (alpha-scissor-threshold, unshaded, filtered) when using a custom material
• In the editor, refresh the screen material every second to show material updates performed in the property editor

Additionally other changes include

• Use a range slider for alpha_scissor_threshold to ensure [0..1] range
• Fixed unshaded option in Godot 4 via shading_mode
• Fixed editor tool failure when clearing scene
• Used dirty-flags approach to updating render objects to minimize unnecessary changes

[Malcolmnixon]

This method of letting the user provide their own material allows for fine control of the rendering - similar to pull request #476:

[BastiaanOlij]

lgtm, this should solve all current and future wishes around controlling the material.

We'll need to document it properly, especially that the albedo texture will always be set by us.

[surreal6]

Hi @Malcolmnixon i'm sorry i have no time to checkout and test it, but i read the code and this solution seems good for me, also with this it's easier to understand the viewport workflow for everyone.

You can also close my PR when you merge this 👍 Thanks!