There's just no good reason to do this.
The query entrypoints already tell us if we need to use
GL_TEXTURE_EXTERNAL_OES for a particular format/modifier. We also have
RGB -> YUV fallbacks which should be able to work well with TEXTURE_2D.
TEXTURE_EXTERNAL pessimises quite hard, forcing GPU-side reloads as well
as bad filtering. Allowing multi-planar formats to use TEXTURE_2D should
thus result in performance and quality improvements.
Signed-off-by: Daniel Stone <daniels@collabora.com>