More changes for Unreal Engine 4 GL4 compatibility. With this change UE4 can be traced/replayed in full-stream mode in both GL3 and GL4 mode, and snapshot/restore works in both modes too (with some query issues I'm still looking at).

- Adding glDispatchCompute, glDispatchComputeIndirect, glBindImageTexture, glCopyImageSubData to replayer whitelist
- Texture snapshot/restore code supports cubemap arrays now, but these paths are not really tested yet
- Adding voglreplay new command line param and replayer object flag to disable front buffer restoring (I may make this the default): --disable_frontbuffer_restore
- Generic state snapshot/restore code now supports GL4 image unit bindings