Per discussion with Olli Etuaho on public_webgl, clarified failure cases in section "Packing Restrictions for Uniforms and Varyings".
Merge pull request #329 from kenrussell/packing

Per discussion with Olli Etuaho on public_webgl, clarified failure cases...
Per discussion on public_webgl, clarified that MAX_TEXTURE_SIZE must be a power of two.
Merge pull request #330 from kenrussell/npot

Per discussion on public_webgl, clarified that MAX_TEXTURE_SIZE must be ...