Add Shader support using DirectX/OpenGL API for video post-processing

by skoruga for VideoLAN

A lot of video post-processing such as RGB-YUV conversion can be accelerated by using the GPU. This can either be achieved through a fragment shader, where it is the last step before a pixel is displayed on the screen, or it can be done with DirectCompute/CUDA/OpenCL. The method will vary depending on the task and intent. I will implement support for shaders by using the DirectX and OpenGL API and write a few shaders such as RGB-YUV conversion.