st/mesa: handle texture_from_pixmap and other surface-based textures correctly

st/mesa: handle texture_from_pixmap and other surface-based textures correctly

There were 2 issues with it:
1) The texture format which should be used for texturing was only set
   in gl_texture_image::TexFormat, which wasn't used for sampler views.
2) Textures are sometimes reallocated under some circumstances
   in st_finalize_texture, which is unacceptable if the texture comes
   from a window system.

The issues are resolved as follows:
1) If surface_based is true (texture_from_pixmap, etc.), store the format
   in a new variable st_texture_object::surface_format.
2) Don't reallocate a surface-based texture in st_finalize_texture.

Also don't use st_ChooseTextureFormat is st_context_teximage, because
the format is dictated by the caller.

This fixes the glx-tfp piglit test.

Reviewed-by: Adam Jackson <ajax@redhat.com>