Fix crash when creating a ParticleSystem of size 0.

This fails when we try to create a SpriteBatch of size 0, which
throws. This is not an error in standard LOVE.

This fixes issue #30.
Remove calls to glGetError in VBO.

glGetError is very slow on NaCl. Some games create many VBOs on the
fly, and each one would call glGetError at least twice.

We now just assume the call succeeds; any errors will be displayed in
the Chrome JavaScript console, and it is unlikely that any games depend
on the behavior of having a VBO creation fail.

This fixes issue #31.