Direct3D: implement HLSL pixel shading in the rendering pipeline

The user can choose to apply a builtin shader using a selection list
in the options of the Direct3D vout module. A custom shader function
can also be loaded by specifying the path of the shader file.

Many changes since the latest patch proposal: we are not compiling
"shader techniques" anymore but now shader functions using the "main"
entrypoint.  All the shaders previously in pixelShader.fx are now
builtins.

Based on the code by Sasha Koruga for GSoC 2010.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>