Changing pipe_resource was wrong, because it can be used by other contexts
at the same time. This fixes the last possible race condition in r300g
that I know of.

This also fixes blitting NPOT compressed textures. Random pixels sometimes
appeared at the right-hand edge of the texture.

Finally, this removes r300_texture_desc::stride_in_pixels. It makes little
sense with sampler views and surfaces being able to override width0, height0,
and the format entirely.