gettextureimage: verify cube map is complete

gettextureimage: verify cube map is complete

According to OpenGL 4.6 spec, section 8.11.4 ("Texture Image Queries"),
relative to errors for GetTexImage, GetTextureImage, and GetnTexImage:

  "An INVALID_OPERATION error is generated by GetTextureImage if the
   effective target is TEXTURE_CUBE_MAP or TEXTURE_CUBE_MAP_ARRAY, and
   the texture object is not cube complete or cube array complete,
   respectively."

This fixes arb_get_texture_sub_image piglit tests.

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>