[3.x] Added material blit

[ghsoares]

This is a WIP pull request, the goal of this feature is to make it easier to use shaders to perform instant compute operations in the GPU, without the need to support compute shaders, that can be used on lower-end devices ("instant" means that the shader runs in sync with the caller thread, so it can freeze the thread for some time if the shader performs expensive calculations). The idea is to blit a source texture into a output texture, using any user-defined shader, being Spatial or Canvas Item shader.

The feature is available as a single VisualServer method call:
VisualServer.material_blit(RID material, RID source_texture, RID output_texture)
The source_texture can be read in a CanvasItem shader as TEXTURE input.

I had some issues to write the code, as I don't understand much of rendering back-end, so the code is a bit messy and I may need some help to complete this feature if relevant. I'm targetting 3.x as it is OpenGL only for now, and I don't have Vulkan support in my machine to write a 4.x version.

Here is a demo run of a simple paint program using this feature:

Demo.mp4

_{Don't mind my drawing skills 😅}

Here is the paint demo project:
Paint.zip

Bugsquad edit: This partially addresses godotengine/godot-proposals#1010.

[ghsoares]

Some other use cases could be procedural texture generation, visual simulations (in my case, a fluid flow simulation) and custom post processing based on previous frames (old-school frame-stacking motion blur, for example).

[ghsoares]

I added the ability to provide a source and output rectangle, the source rectangle is used to create a custom projection matrix to read from a specific area of the source texture, and the output rectangle is used in glViewport and glScissor to write to a specific area of the output texture.

There is a demo of it in action:

Demo2.mp4

There is the updated demo project:
Paint.zip