windowManager: shade the actor, not the texture

Applying the "dim window" effect to the MetaWindowActor has two avantages:
first it avoids triggering bugs where ClutterOffscreenEffect doesn't handle
clone paint correctly. Second, it avoids showing the window as dimmed in
alt-Tab and the overview, which is weird.

The small downside of this is that the shadow becomes slightly gray when
the window dimmed, which is wrong - if we switched from blending with gray
to a combination of desaturation and darkening, this problem wouldn't
happen.

Revert out the addition of startY to the shader, since we don't need it
and fix the application of alpha, since we need to handle alpha correctly
for the shadow.

https://bugzilla.gnome.org/show_bug.cgi?id=659634

data/shaders/dim-window.glsl

@@ -1,6 +1,5 @@
 #version 110
 uniform sampler2D tex;
-uniform float startY;
 uniform float fraction;
 uniform float height;
 const float c = -0.2;
@@ -17,14 +16,12 @@ void main()
   float y = height * cogl_tex_coord_in[0].y;
 
   // To reduce contrast, blend with a mid gray
-  cogl_color_out = color * contrast - off * c;
+  cogl_color_out = color * contrast - off * c * color.a;
 
-  // We only fully dim at a distance of BORDER_MAX_HEIGHT from startY and
+  // We only fully dim at a distance of BORDER_MAX_HEIGHT from the top and
   // when the fraction is 1.0. For other locations and fractions we linearly
   // interpolate back to the original undimmed color, so the top of the window
   // is at full color.
-  cogl_color_out = color + (cogl_color_out - color) * max(min((y - startY) / border_max_height, 1.0), 0.0);
+  cogl_color_out = color + (cogl_color_out - color) * max(min(y / border_max_height, 1.0), 0.0);
   cogl_color_out = color + (cogl_color_out - color) * fraction;
-
-  cogl_color_out *= color.a;
 }

js/ui/windowManager.js

@@ -31,16 +31,15 @@ function getTopInvisibleBorder(metaWindow) {
     return outerRect.y - inputRect.y;
 }
 
-function WindowDimmer(actor, window) {
+function WindowDimmer(actor) {
-    this._init(actor, window);
+    this._init(actor);
 }
 
 WindowDimmer.prototype = {
-    _init: function(actor, window) {
+    _init: function(actor) {
         this.effect = new Clutter.ShaderEffect({ shader_type: Clutter.ShaderType.FRAGMENT_SHADER });
         this.effect.set_shader_source(getDimShaderSource());
 
-        this.window = window;
         this.actor = actor;
     },
 
@@ -52,7 +51,6 @@ WindowDimmer.prototype = {
         }
 
         if (fraction > 0.01) {
-            Shell.shader_effect_set_double_uniform(this.effect, 'startY', getTopInvisibleBorder(this.window));
             Shell.shader_effect_set_double_uniform(this.effect, 'height', this.actor.get_height());
             Shell.shader_effect_set_double_uniform(this.effect, 'fraction', fraction);
 
@@ -71,11 +69,11 @@ WindowDimmer.prototype = {
     _dimFraction: 0.0
 };
 
-function getWindowDimmer(texture, window) {
+function getWindowDimmer(actor) {
-    if (!texture._windowDimmer)
+    if (!actor._windowDimmer)
-        texture._windowDimmer = new WindowDimmer(texture, window);
+        actor._windowDimmer = new WindowDimmer(actor);
 
-    return texture._windowDimmer;
+    return actor._windowDimmer;
 }
 
 function WindowManager() {
@@ -270,30 +268,28 @@ WindowManager.prototype = {
         let actor = window.get_compositor_private();
         if (!actor)
             return;
-        let texture = actor.get_texture();
         if (animate)
-            Tweener.addTween(getWindowDimmer(texture, window),
+            Tweener.addTween(getWindowDimmer(actor),
                              { dimFraction: 1.0,
                                time: DIM_TIME,
                                transition: 'linear'
                              });
         else
-            getWindowDimmer(texture, window).dimFraction = 1.0;
+            getWindowDimmer(actor).dimFraction = 1.0;
     },
 
     _undimWindow: function(window, animate) {
         let actor = window.get_compositor_private();
         if (!actor)
             return;
-        let texture = actor.get_texture();
         if (animate)
-            Tweener.addTween(getWindowDimmer(texture, window),
+            Tweener.addTween(getWindowDimmer(actor),
                              { dimFraction: 0.0,
                                time: UNDIM_TIME,
                                transition: 'linear'
                              });
         else
-            getWindowDimmer(texture, window).dimFraction = 0.0;
+            getWindowDimmer(actor).dimFraction = 0.0;
     },
 
     _mapWindow : function(shellwm, actor) {