### Godot version

2a0aef5f09

### System information

Windows 11, Ryzen 9 5950X, RTX 3090 (531.79) Vulkan, 64GB RAM

### Issue description

The issue occurs due to two things.
When textures/images are imported, the `fix_alpha_edges` / `fix_alpha_borders` function in the importer will modify the color values but leaves the alpha values unchanged.

Later, when using the `_convert` function in the `image.cpp` file or `Image::convert` to convert from `Image::FORMAT_RGBA` to `Image::FORMAT_L8` the following image results (raw on top, conversion on bottom).
![Clipboard - May 22, 2023 8_13 PM](https://github.com/godotengine/godot/assets/5207577/52f4597e-17b6-48c5-8fb4-d6d37012d8f4)

I, originally, thought that the `fix_alpha_edges` / `fix_alpha_borders` code was at fault but it turns out to not necessarily be the case.  I modified the `_convert` function in `image.cpp` to calculate grayscale values correctly and to consider the alpha channel or not and now I'm getting the expected output as shown here:
![Clipboard - May 23, 2023 7_16 AM](https://github.com/godotengine/godot/assets/5207577/73babb20-22bb-4f23-808f-5d49dce55f2d)


### Steps to reproduce

Import `Image::FORMAT_RGBA` image into any project.

Convert image in code by calling the `convert` function to `Image::FORMAT_L8`.

Screen shot of MRP here:
![image](https://github.com/godotengine/godot/assets/5207577/9d7d0078-5e41-45d1-b5c5-fc291b839fa9)


### Minimal reproduction project

Attached now
[Issue_77393.zip](https://github.com/godotengine/godot/files/11545149/Issue_77393.zip)

case FORMAT_RGBAH:
				if (p_renormalize) {
					_generate_po2_mipmap<uint16_t, 4, true, Image::average_4_half, Image::renormalize_half>(reinterpret_cast<const uint16_t *>(&wp[prev_ofs]), reinterpret_cast<uint16_t *>(&wp[ofs]), prev_w, prev_h);
				} else {
					_generate_po2_mipmap<uint16_t, 4, false, Image::average_4_half, Image::renormalize_half>(reinterpret_cast<const uint16_t *>(&wp[prev_ofs]), reinterpret_cast<uint16_t *>(&wp[ofs]), prev_w, prev_h);
				}
				break;
			case FORMAT_RGBE9995:
				if (p_renormalize) {
					_generate_po2_mipmap<uint32_t, 1, true, Image::average_4_rgbe9995, Image::renormalize_rgbe9995>(reinterpret_cast<const uint32_t *>(&wp[prev_ofs]), reinterpret_cast<uint32_t *>(&wp[ofs]), prev_w, prev_h);
				} else {
					_generate_po2_mipmap<uint32_t, 1, false, Image::average_4_rgbe9995, Image::renormalize_rgbe9995>(reinterpret_cast<const uint32_t *>(&wp[prev_ofs]), reinterpret_cast<uint32_t *>(&wp[ofs]), prev_w, prev_h);
				}
				break;
			default: {
			}
		}
		prev_ofs = ofs;
		prev_w = w;
		prev_h = h;
	}
	mipmaps = true;
	return OK;
}
Error Image::generate_mipmap_roughness(RoughnessChannel p_roughness_channel, const Ref<Image> &p_normal_map) {
	Vector<double> normal_sat_vec; 
	double *normal_sat = nullptr; 
	int normal_w = 0, normal_h = 0;
	ERR_FAIL_COND_V_MSG(p_normal_map.is_null() || p_normal_map->is_empty(), ERR_INVALID_PARAMETER, "Must provide a valid normal map for roughness mipmaps");
	Ref<Image> nm = p_normal_map->duplicate();