Move 1.0.x latest spec into a subdirectory.

Once WebGL 2.0 exists, we'll have latest/2.0 and latest/1.0. For now
add a redirect from latest/ into latest/1.0.
Fix relative links to ../../../resources.
Merge pull request #357 from grorg/webgl2-move-1.0-spec

Move 1.0.x latest spec into a subdirectory.