fixes triangle winding direction for ofIcoSphere

triangles were wound the other way around, which resulted in backfaces to be rendered instead of front faces when glEnable(GL_CULL_FACE), glCullFace(GL_BACK) was applied.

This reverses the triangle winding mode and fixes it.

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>
uses std::swap to swap indices

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>
Merge pull request #2267 from tgfrerer/fix-icosphere-triangle-winding

fixes triangle winding direction for ofIcoSphere