effects/desktopgrid: Fix grid's size and scale w.r.t. row/column spacing

This patch makes grid actually account for the spacing, so that when
fully shown it won't cut off bottom or right edge of desktops. It can
be easily seen with a wallpaper that has visible features on the edges
such as borders, or by settings rowSpacing & columnSpacing to an
absurdly high value.

src/effects/desktopgrid/qml/main.qml

@@ -154,7 +154,7 @@ Rectangle {
         id: grid
 
         property Item currentItem
-        readonly property real targetScale: 1 / Math.max(rows, columns)
+        readonly property real targetScale: Math.min(parent.width / width, parent.height / height)
         property real panelOpacity: 1
 
         Behavior on x {
@@ -186,8 +186,8 @@ Rectangle {
             }
         }
 
-        width: parent.width * columns
-        height: parent.height * rows
+        width: (parent.width + columnSpacing) * columns - columnSpacing
+        height: (parent.height + rowSpacing) * rows - rowSpacing
         rowSpacing: PlasmaCore.Units.gridUnit
         columnSpacing: PlasmaCore.Units.gridUnit
         rows: container.effect.gridRows