rebind instance data buffer if index buffer changes
rebind instance data buffer if index buffer changes

Fix case of drawing different indices of same vertex buffer using same
instance data.

This can happen in the case of conditionally drawing subsets of vertex
buffer stored in different index buffers while re-using non-transient
instance buffers.

LWJGL-CI/bgfx,bkaradzic/bgfx,bkaradzic/bgfx,jdryg/bgfx,emoon/bgfx,LWJGL-CI/bgfx,jdryg/bgfx,LWJGL-CI/bgfx,jpcy/bgfx,emoon/bgfx,mendsley/bgfx,emoon/bgfx,bkaradzic/bgfx,LWJGL-CI/bgfx,jdryg/bgfx,mendsley/bgfx,jpcy/bgfx,jpcy/bgfx,jdryg/bgfx,bkaradzic/bgfx,jpcy/bgfx,mendsley/bgfx