The matrix uniform parameter of newly created ShaderMaterial is zero matrix instead of identity matrix

[beicause]

Tested versions

4.4, 4.5

System information

Godot v4.4.stable.mono - EndeavourOS SMP PREEMPT_DYNAMIC Fri, 09 May 2025 17:36:18 +0000 on Wayland - X11 display driver, Multi-window, 1 monitor - Vulkan (Mobile) - integrated AMD Radeon Graphics (RADV RENOIR) - AMD Ryzen 7 4800U with Radeon Graphics (16 threads)

Issue description

mat2/mat3/mat4 uniform parameter of newly created ShaderMaterial is zero matrix instead of identity matrix.

Video_2025-05-18_21-48-38.mp4

Steps to reproduce

1. Create a ShaderMaterial
2. Load a shader file with mat2/mat3/mat4 uniform
3. Then the uniform parameter is actually zero matrix though it's identity matrix in the inspector, unless you reload the scene or modify the uniform parameter

Minimal reproduction project (MRP)

shader-mat-default-value.zip

[beicause]

This issue also happends in game. This behavior may be expected or not, because the printed default value is an identity matrix:

extends Node2D

func _ready() -> void:
	var shader:=load("res://main.gdshader")
	var sp:=Sprite2D.new()
	sp.texture=load("res://icon.svg")
	var sm:=ShaderMaterial.new()
	sm.set_shader(shader)
	print("parameter is: ",sm.get_shader_parameter("m"))
	print("default parameter is: ",RenderingServer.shader_get_parameter_default(shader.get_rid(),"m"))
	#sm.set_shader_parameter("m",Transform2D.IDENTITY)
	sp.material=sm
	add_child(sp)

#parameter is: <null>
#default parameter is: [X: (1.0, 0.0), Y: (0.0, 1.0), O: (0.0, 0.0)]

shader-mat-default-value (2).zip

[beicause]

I've thought several solutions:

1. Set shader parameters to their default values (identity matrix) in Material::set_shader only in the editor.
2. Change _fill_std140_ubo_empty to fill the matrix with an identity matrix instead of zero.
3. Change MaterialStorage::shader_get_parameter_default to return a zero matrix instead of identity.

I think the first solution is better. The other two would change the existing behavior in games and it's not worthwhile.