mesa: Add error condition for integer formats in glGetTexImage()

mesa: Add error condition for integer formats in glGetTexImage()

OpenGL 4.0 spec, page 306 suggests an INVALID_OPERATION in glGetTexImage
if :
   "format is one of the integer formats in table 3.3 and the internal
    format of the texture image is not integer, or format is not one of
    the integer formats in table 3.3 and the internal format is integer."

V2: Use helper function _mesa_is_format_integer()

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>