mesa: add support for GL_OES_EGL_image_external

mesa: add support for GL_OES_EGL_image_external

This is an OpenGL ES specific extension.  External textures are textures that
may be sampled from, but not be updated (no glTexSubImage* and etc.).  The
image data are taken from an EGLImage.

Reviewed-by: Brian Paul <brianp@vmware.com>
Acked-by: Jakob Bornecrantz <jakob@vmware.com>