Expose RenderSceneBuffers(RD) through ClassDB

[BastiaanOlij]

In preparations of planned work around exposing some internals of the rendering engine, this PR completes the bind methods implementation of the RenderSceneBuffer and RenderSceneBufferRD objects.

Q: Should we rename RenderSceneBufferRD to RDRenderSceneBuffer to be inline with the other exposed rendering device objects?

[BastiaanOlij]

I'm also wondering if we should remove if we should remove the GDVIRTUAL implementation from RenderSceneBuffers and turn that into a pure virtual class, and instead implement a RenderSceneBuffersExtension class to be used when a renderer is fully implemented in GDExtension.

[BastiaanOlij]

Decided to split off RenderSceneBuffersExtension as a base class to implement RenderSceneBuffers in GDExtension. Unlikely to be used any time soon but handy to have for the future.

Also decided to keep the RenderSceneBuffersRD name.

[BastiaanOlij]

Not sure what Schemas validity error : Element 'class', attribute 'version': The attribute 'version' is not allowed. is all about, I'm guessing this has to do with us still being in 4.1?

edit rebasing helped :)

[BastiaanOlij]

After discussing with @reduz on contributors chat, decided to register RenderSceneBuffersRD as an abstract class so it can't be instantiated by users. This is always created internally in the rendering engine.

[BastiaanOlij]

In order to make this more easily usable I've exposed get_color_texture and get_depth_texture methods that will return either the MSAA variants (when applicable) or normal variants that the extensions most like will be using. Access to underlying buffers is possible through the get_texture function. We can further extend these methods once we implement 2D HDR and in that mode potentially render directly into the render targets buffer.

In order to make this work I've moved the MSAA buffer creation back into the RenderSceneBufferRD implementation as this in hindsight makes far more sense and the code between mobile and forward+ is nearly identical.
The only difference is that while TEXTURE_USAGE_SAMPLING_BIT is used, it was omitted from the texture_is_format_supported_for_usage check in the forward+ renderer.
This needs further investigation before we merge this as this may render DATA_FORMAT_D24_UNORM_S8_UINT unusable.

I've also added access to the velocity texture.

[Zireael07]

I've also added access to the velocity texture.

YAAAY!

[clayjohn]

Great work so far! I left some comments.

[reduz]

ah I approved by accident, it looks good but I would change this specifically.

was added by mistake

[clayjohn]

Looks good to me too!

[YuriSizov]

Thanks!