opengl: interop: remove usage of vlc_gl_api_t and move to core

The rationale for removing vlc_gl_api_t is that interop implementations are already required to load specific function for most of them (like the eglCreateImageKHR function) and having vlc_gl_api_t in the public module API require exposing the OpenGL vtable and types to the public scope, which is pretty hard to justify and port/maintain for every platform.

By removing it, it makes it easy to move interop definition to the core and makes it available for out-of-tree modules.