Native bindings to the OES_EGL_image_external extension.

This extension provides a mechanism for creating EGLImage texture targets from EGLImages. This extension defines a new texture target, TEXTURE_EXTERNAL_OES. This texture target can only be specified using an EGLImage. There is no support for most of the functions that manipulate other texture targets (e.g. you cannot use gl*Tex*Image*() functions with TEXTURE_EXTERNAL_OES). Also, TEXTURE_EXTERNAL_OES targets never have more than a single LOD level. Because of these restrictions, it is possible to bind EGLImages which have internal formats not otherwise supported by OpenGL ES. For example some implementations may allow EGLImages with planar or interleaved YUV data to be GLES texture target siblings. It is up to the implementation exactly what formats are accepted.