Feature: #183 custom window shader & #193 --no-fading-destroyed-argb

- Add --glx-fshader-win, which specifies a custom fragment shader for
  painting windows. compton-default-fshader-win.glsl is the shader with
  default behavior, and compton-fake-transparency-fshader-win.glsl
  provides a template of fake transparency. (#183)

- Add --force-win-blend to force all windows to be painted with
  blending.

- Add --no-fading-destroyed-argb, as a workaround of bugs in some WMs.
  (#193)