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)

compton-default-fshader-win.glsl

@@ -0,0 +1,11 @@
+uniform float opacity;
+uniform bool invert_color;
+uniform sampler2D tex;
+
+void main() {
+	vec4 c = texture2D(tex, gl_TexCoord[0]);
+	if (invert_color)
+		c = vec4(vec3(c.a, c.a, c.a) - vec3(c), c.a);
+	c *= opacity;
+	gl_FragColor = c;
+}