MOUSE6 and MOUSE7 keys missing

[slonkazoid]

these keys were present in latest tag, but do not work anymore

[mikejsavage]

We switched from GLFW to SDL which doesn't expose anything beyond MOUSE5. IIRC it unofficially supports mouse6/7 anyway, and I think qfusion does too, but none of us have a 7 button mouse to test it.

I can push a speculative fix later, or if you want to mess around the relevant code is https://github.com/mikejsavage/cocainediesel/blob/master/source/client/cl_sdl.cpp#L283

[mikejsavage]

8dcd17f