VideoCommon: Merge LineGeometryShader into GeometryShaderGen.

This adds line-width emulation support to OpenGL, with stereoscopy support to boot!

### To-Do
- [x] Disable the geometry shader stage if it is not needed.
- [x] Check for geometry shader support in the API (OpenGL ES).
May you also add this to the top? Mesa allows to have glsl330 with gl3.0 and disable the geometry shader in this case...
g_Config.backend_info.bSupportsGeometryShaders = GLExtensions::Version() >= 320;