softpipe: Split 3D to 2D coords conversion into separate function

softpipe: Split 3D to 2D coords conversion into separate function

This is to avoid tying the conversion to the sampling -
textureQueryLod will need to do the conversion too, but it does not do
any sampling.

So instead of a "get_samples" vfunc, there is just a bool saying
whether the conversion is needed or not. This solution keeps a nice
property of not adding any overhead for the common case (2D textures).

v2:
  - replaced the "convert_coords" vfunc with a "need_cube_convert"
    boolean to avoid overhead of copying arrays in common case
  - removed an unused typedef
  - splitted too long lines in convert_cube
  - const fixes in convert_cube

Reviewed-by: Brian Paul <brianp@vmware.com>