A couple of fixes

Fixes a couple of issues/warnings.

Clang/LLVM warned about the unused private member, the rest are a couple of functions marked as unused by Cppcheck version 1.66. 

I'm slightly wary of the change I did in RenderDevice.h which changes the interface and which consequences this might have. Would be nice if someone could doublecheck whether the function is safe to remove from the interface. From what I've seen on the bug tracker there has been some talk of other renderers, most notably opengl, where I guess this part of the interface might be in use.

In addition to the functions removed here, Cppcheck also warns about channel_finished in SoundManager.cpp being unused, but that seems to be passed into Mix_ChannelFinished() at line 246.
Therefore I haven't touched it, might be a false positive where cppcheck isn't able to spot the usage.