Support for mesh primitive modes LINE_LOOP

[atteneder]

Out of all mesh primitive modes, these are not supported and render incorrectly:

• LINE_LOOP
• TRIANGLE_STRIP
• TRIANGLE_FAN

For the latter two, there's issue #316

Unity does not have native support for those (see MeshTopology), so what needs to be done is calculate new index buffers instead of using the provided originals.

As an example, in case of line loop the calculated index buffer needs to be increased by one element in size, which is a duplicate of the first index (to close the loop).