SwitchPanel: fix app icons opacity combination

For some apps the background is not displayed properly, showing
some kind of shawow around the app icon like for example firefox
or skype. This is due to an issue in combining alpha channels.

src/switchpanel.c

@@ -158,6 +158,7 @@ static void changeImage(WSwitchPanel *panel, int idecks, int selected, Bool dim,
 			color.red = WMRedComponentOfColor(panel->gray) >> 8;
 			color.green = WMGreenComponentOfColor(panel->gray) >> 8;
 			color.blue = WMBlueComponentOfColor(panel->gray) >> 8;
+			color.alpha = 255;
 			RFillImage(back, &color);
 		}