2D: Fix rendering artifacts when using BackBufferCopy.

[stuartcarnie]

Closes #101686
Addresses feedback from PR #101931 by @Repiteo

Instance buffer usage was being reset after calling _render_batch, rather than at the end of the draw pass in canvas_render_items for both the GLES3 and RendererRD implementations. This PR fixes both.

Before using MRP, notice artefacts

before	after

[clayjohn]

Should we remove the clear from the early-out as well then?

godot/drivers/gles3/rasterizer_canvas_gles3.cpp

Lines 664 to 667 in b15b24b

	// Nothing to render, just return.
	state.current_batch_index = 0;
	state.canvas_instance_batches.clear();
	return;

[stuartcarnie]

Should we remove the clear from the early-out as well then?

godot/drivers/gles3/rasterizer_canvas_gles3.cpp

Lines 664 to 667 in b15b24b

	// Nothing to render, just return.
	state.current_batch_index = 0;
	state.canvas_instance_batches.clear();
	return;

Probably should not be setting current_batch_index to 0 either

[stuartcarnie]

Thanks – good catch @clayjohn. I've removed that early-exit code that resets the state in the gles3 driver.

[Repiteo]

Thanks!

[mihe]

As best I can tell, this seems to have broken CanvasItem.clip_children: #102147