gallium: Add index bias parameter to draw_elements and friends.

gallium: Add index bias parameter to draw_elements and friends.

indexBias corresponds to:
- BaseVertexIndex parameter of D3D9's
  IDirect3DDevice9::DrawIndexedPrimitive method
- BaseVertexLocation parameter of ID3D10Device::DrawIndexed

Although a positive indexBias can be easily be implemented in Gallium by
adding indexBias*stride to each vertex buffer base offset, a negative
indexBias cannot, as the final vertex buffer offset could be negative.

I'm not aware of this functionality being exposed to GL drivers, so for
now all hardware drivers will just assert(indexBias == 0).

See also:
- http://msdn.microsoft.com/en-us/library/bb174369.aspx (D3D9)
- http://msdn.microsoft.com/en-us/library/ff556126.aspx (D3D10 DDI)