Exporting from preview 2d gets rid of alpha channel

[mieldepoche]

Material Maker version:
1.4a2

OS/device including version:
Windows 11

Issue description:

When quick-exporting an image via the preview2d menu, it looks like it doesn't take transparency into account:

preview	exported file

Steps to reproduce:

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[karmaral]

Just stopping by to confirm this, came looking for the same thing.

[karmaral]

I've been poking around the codebase to see if I could find the problem, and arrived to this function:

func in_thread_get_texture() -> Texture2D:
	if texture_needs_update:
		var byte_data : PackedByteArray = rd.texture_get_data(rid, 0)
		var image_format : Image.Format
		match texture_format:
			RenderingDevice.DATA_FORMAT_R32_SFLOAT:
				image_format = Image.FORMAT_RF
			RenderingDevice.DATA_FORMAT_R32G32B32A32_SFLOAT:
				image_format = Image.FORMAT_RGBAF
			RenderingDevice.DATA_FORMAT_R16_SFLOAT:
				image_format = Image.FORMAT_RH
			RenderingDevice.DATA_FORMAT_R16G16B16A16_SFLOAT:
				image_format = Image.FORMAT_RGBAH
			RenderingDevice.DATA_FORMAT_R8G8B8A8_UNORM:
				image_format = Image.FORMAT_RGBA8
		var image : Image = Image.create_from_data(texture_size.x, texture_size.y, false, image_format, byte_data)
		texture.set_image(image)
		texture_needs_update = false
	return texture

When exporting from the Preview2D, it seems to match RenderingDevice.DATA_FORMAT_R16G16B16A16_SFLOAT.

Looking around Godot's codebase, it seems it doesn't really check alpha for formats beyond 8 bit. See image_to_png and Image::detect_alpha.

To confirm this, if you force the format to Image.FORMAT_RGBA8 before exporting, the outputted image properly maintains the alpha channel.

	...
	var image : Image = Image.create_from_data(texture_size.x, texture_size.y, false, image_format, byte_data)
	image.convert(Image.FORMAT_RGBA8) # force 8bit
	texture.set_image(image)
	texture_needs_update = false
return texture

I have little experience in image processing and I'm not familiar with the codebase, so I have no idea what potential issues narrowing the image format like this could cause in the future.

Maybe there's a way of indicating somewhere that this is a PNG export and should clamp everything to 8bit?