Fixes vertex allocation in the bgfx nanovg backend

Vertex allocation would be insufficient if gl->nverts+n was bigger than the next power-of-two level. This change actually allocates the POT level that is sufficient for the required amount of vertices, not just twice the previous one.

Attached image of the bug caught red-handed.

![nanovg bgfx POT bug](https://cloud.githubusercontent.com/assets/1451391/4253888/52eeb674-3aa5-11e4-937d-1f38351b155e.png)