[4/5] WineD3D: Merge indexbuffer and buffer implementations

[Stefan Dösinger]

The bind_count is increased for SetIndices as well, that in theory enables 
converting index formats. Since we don't need any conversion on any of the 
available index buffer formats the conversion table is only built once(and 
ends up empty), similarly to d3d7 vertex buffers. (That's just for 
performance, we could rebuild it every use, but its just not needed).