Patch: Middle button scroll emulation

[Zuu]

I'm surprised if many people have use of this patch. This patch is usefull if you use a 3-button input device without a scroll wheel. Such as a second generation Wacom Intous.

The patch adds a feature which emulates scroll when you hold the middle button down and moves your pen/mouse/whatever.

This patch is implemented in src/video/sdl_v.cpp so win32 users will not be able to use it. (I guess win32 use src/video/win32_v.cpp for input and not sdl_v.cpp)

This is basically a hack. It's not meant to be included in the trunk as I don't think people who don't need it want to have it. It will just annoy those people.