mesa: Allow external textures to use fallback (0, 0, 0, 1)

mesa: Allow external textures to use fallback (0, 0, 0, 1)

Fixes GL2ExtensionTests/egl_image_external/TestSimpleUnassociated.test
which is part of gles2/3 conformance suite. Here image external
textures are switched to be treated the same as 2D textures. These
can be associated with the fallback texture providing fixed sample
values of (0, 0, 0, 1).

The OES_EGL_image_external spec says:

  "Sampling an external texture which is not associated with any EGLImage
   sibling will return a sample value of (0,0,0,1)."

  "External textures cannot be used with TexImage2D, TexSubImage2D,
   CompressedTexImage2D, CompressedTexSubImage2D, CopyTexImage2D, or
   CopyTexSubImage2D, and an INVALID_ENUM error will be generated if
   this is attempted."

And quoting Chad:

  "That's enforced in _mesa_TexImage*() by calling
   legal_teximage_target(), and enforced in _mesa_TexSubImage*() by
   calling legal_texsubmimage_target(). Each of the
   legal_tex*image_target() functions reject external textures.
   Therefore, allowing GL_TEXTURE_EXTERNAL_OES in store_texsubimage()
   won't violate the above spec quote.

   I think it's safe to allow GL_TEXTURE_EXTERNAL_OES in
   store_texsubimage(), as long as the texture has only a single
   plane. Luckily, that's the only type of external textures that
   Mesa currently supports."

CC: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>