Fixies for OpenAL, Visual C++ compiling, retexturing, mouse input

OpenAL
- Tutorial of official effects extension guide uses alGetProcAddress to get extensions.

Visual C++
- Microsoft SDK's GL/gl.h header does not include automatically the windows.h header.
- Uninitialized variable generates break for debugger. Since there are to many calls of such code it's hard to debug it.

Mouse Input
- Mouse cursor position updates even when console or menu active, the game is paused.

Retexturing
- Original sky textures are in pcx and tga formats. When the game compiled without retexturing support it's cann't load sky texture because tga files are prohibited.