getteximage: assume texture image is empty for non defined levels

Current code is returning an INVALID_OPERATION when trying to use
getTextureImage() on a level that has not been explicitly defined.

That is, we define a mipmapped Texture2D with 3 levels, and try to use
GetTextureImage() for the 4th levels, and INVALID_OPERATION is returned.

Nevertheless, such case is not listed as an error in OpenGL 4.6 spec,
section 8.11.4 ("Texture Image Queries"), where all the case errors for
this function are defined. So it seems this is a valid operation.

On the other hand, in section 8.22 ("Texture State and Proxy State") it
states:

  "Each initial texture image is null. It has zero width, height, and
   depth, internal format RGBA, or R8 for buffer textures, component
   sizes set to zero and component types set to NONE, the compressed
   flag set to FALSE, a zero compressed size, and the bound buffer
   object name is zero."

We can assume that we are reading this initialized empty image when
calling GetTextureImage() with a non defined level.

With this assumption, we will reach one of the other error cases defined
for the functions. In the end this means that we would end up returning
INVALID_VALUE to the caller.

This fixes arb_get_texture_sub_image piglit tests.

v2: just return INVALID_VALUE if there is no defined level (Iago)

Signed-off-by: Juan A. Suarez Romero <[email protected]>
Reviewed-by: Iago Toral Quiroga <[email protected]>

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