Support for vs_1_x with recent DX SDKs, but old gfx cards. This way the shaders are still compiled to vs_1_x, but need an extra dx dll. Hence it's not enabled by default, but may be opted in with the IrrCompileConfig _IRR_D3D_USE_LEGACY_HLSL_COMPILER

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@2494 dfc29bdd-3216-0410-991c-e03cc46cb475