getteximage: Return correct error value when texure object is not found

getteximage: Return correct error value when texure object is not found

glGetTextureSubImage() and glGetCompressedTextureSubImage() are currently
returning INVALID_OPERATION error when the passed texture argument does not
correspond to an existing texture object. However, the error should be
INVALID_VALUE instead. From OpenGL 4.5 spec PDF, section '8.11. Texture
Queries', page 236:

    "An INVALID_VALUE error is generated if texture is not the name of
     an existing texture object."

Same wording applies to the compressed version.

The INVALID_OPERATION error is coming from the call to
_mesa_lookup_texture_err(). This patch uses _mesa_lookup_texture() instead
and emits the correct error in the caller.

Fixes: GL45-CTS.get_texture_sub_image.errors_test
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>