### Godot version

v4.0.beta17.official [c40020513]

### System information

Windows 11, OpenGL (Compatibility), RTX 2060 (31.0.15.2824)

### Issue description

I tried to make a blurred panel for a ui element with texturelod via shader:

```glsl
shader_type canvas_item;

uniform sampler2D SCREEN_TEXTURE : hint_screen_texture, filter_linear_mipmap_anisotropic;
uniform float blur_amount : hint_range(0.0, 5.0);

void fragment() {
	vec4 blurred = textureLod(SCREEN_TEXTURE, SCREEN_UV, blur_amount);
	COLOR = blurred;
}
```

but what I got instead is this:
![image](https://user-images.githubusercontent.com/72286622/217170426-49f6712d-47ef-4c53-b9f1-30d8ad868084.png)

vulkan renderer does not exhibit this issue:
![image](https://user-images.githubusercontent.com/72286622/217170565-a013c712-94f6-49d1-b9ec-091c28a7790b.png)


### Steps to reproduce

1. Add a TextureRect node
2. Add a Panel node to overlay the previously added node
3. Apply the blur shader to the panel node

### Minimal reproduction project

N/A
ERR_FAIL_COND_MSG(rt->backbuffer_fbo != 0, "Cannot allocate RenderTarget backbuffer: already initialized.");
	ERR_FAIL_COND(rt->direct_to_screen);
	
	
	int count = MAX(1, Image::get_image_required_mipmaps(rt->size.x, rt->size.y, Image::FORMAT_RGBA8) - 4);
	if (rt->size.x > 40 && rt->size.y > 40) {
		GLsizei width = rt->size.x;
		GLsizei height = rt->size.y;
		rt->mipmap_count = count;
		glGenTextures(1, &rt->backbuffer);
		glBindTexture(GL_TEXTURE_2D, rt->backbuffer);
		uint32_t texture_size_bytes = 0;
		for (int l = 0; l < count; l++) {
			texture_size_bytes += width * height * 4;
			glTexImage2D(GL_TEXTURE_2D, l, rt->color_internal_format, width, height, 0, rt->color_format, rt->color_type, nullptr);
			width = MAX(1, (width / 2));
			height = MAX(1, (height / 2));
		}