Add the validation and entrypoints for
ANGLE_stream_producer_d3d_texture_nv12, which is a new EGL extension
currently being written. The purpose of this extension is to allow
insertion of D3D11 NV12 textures to be inserted into a stream. This acts
as the producer of the EGL stream.

BUG=angleproject:1332

Change-Id: I50d4565cc82b63f7da7632adad4ac4c77d8800f2
Reviewed-on: https://chromium-review.googlesource.com/336695Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ian Ewell <ewell@google.com>