Implement render_target_was_used API so that Viewports can properly check if they have been used.

[clayjohn]

Fixes: #55471

In GLES3 we have to iterate over textures to bind them so at the same time we can just check if the textures are render targets and set them to used. In the RD renderer we need to carry a reference to the render targets as we bake the textures into a uniform set and only use that at draw time.

Also fixes a bug in the GLES3 renderer that caused TEXTURE to override the first texture uniform.

Testing in all three renderers and using the Viewport as a TEXTURE, a uniform (in both canvas item and spatial shaders), and as a child of subviewportcontainer

cc @Sauermann

[Sauermann]

I have tested this PR and it solves the following problem for me:
When using a SubViewport with render_target_update_mode = UPDATE_WHEN_VISIBLE as a texture for a TextureRect, then the TextureRect is displayed completely black.

In my setting I was unable to recreate the Uniforms were never supplied for set (3) at the time of drawing, which are required by the pipeline problem of the linked PR (not sure, how to do that).

[clayjohn]

For the RD renderer, this does not work for Viewports used in scene shaders yet as they cache their uniform sets both in the MaterialData struct and in the surface cache. I don't want to pass a reference to the render targets in the surface cache as we need to keep it as small as possible.

I think I have an idea to get around this. Will try it out tomorrow.

[clayjohn]

Should be ready for review and merging now.

[akien-mga]

Thanks!