While for normal textures, GL_TEXTURE_2D should be used, when it's an external
texture, binding it using GL_TEXTURE_2D results in an error.
Reading the specification for GL_TEXTURE_EXTERNAL_OES it is unclear whether
getting pixel data from a texture is possible, and tests show it doesn't result
in any data, but in case it would eventually start working, at least bind the
correct target for now.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/362
c681ccef3c