Add ExternalTexture to Godot 4

[vvwx]

Recently I've noticed that ExternalTexture is absent from Godot 4, and I cannot find anything related to it under issues, proposals, and discussions.

It looks like the project converter from Godot 3 -> Godot 4 maps ExternalTexture to ImageTexture (seen here), however it looks like ImageTexture doesn't do what ExternalTexture does in 3.x (granted a lot has changed so maybe I'm missing something).

I was considering submitting this as a bug with the regression tag, but figured I'd start this as a discussion first.

Here is the original PR

[xix-xeaon]

According to the docs in Godot 3, ExternalTexture is based on an extension to OpenGL ES and only supported on Android. Since Godot 4 is all about Vulcan that's probably why it was removed. Of course it might eventually get put back in since @clayjohn is working on adding back GLES3 but I don't know, it's probably not a priority.

[queezle42]

The feature should, in theory, be usable on Linux. The related OpenGL ES extensions (GL_OES_EGL_image_external for GLES2 and GL_OES_EGL_image_external_essl3 for GLES3) are available on Linux and I am using them to intercept- and modify Wayland windows.

If the feature gets added to Godot 4 I'd like to use it to render desktop applications inside of Godot.