conference: avoid excessive image caching during resize events

Throttle avatar image queries to the image provider when the
component size changes.

Gitlab: #466
Change-Id: Ie619671a593bcca75899f3d97c2af2071c427ab4

src/commoncomponents/Avatar.qml

@@ -29,6 +29,7 @@ Item {
 
     enum Mode { Account, Contact, Conversation }
     property int mode: Avatar.Mode.Account
+    property alias sourceSize: image.sourceSize
 
     property string imageId
 

src/mainview/components/ParticipantOverlay.qml

@@ -171,8 +171,15 @@ Item {
         id: avatar
 
         anchors.centerIn: parent
-        height:  Math.min(parent.width / 2, parent.height / 2)
-        width:  Math.min(parent.width / 2, parent.height / 2)
+
+        property real size: Math.min(parent.width / 2, parent.height / 2)
+        height:  size
+        width:  size
+
+        // round the avatar source size up to some nearest multiple
+        readonly property real step: 96
+        property real imageSize: Math.floor((size + step - 1) / step) * step
+        sourceSize: Qt.size(imageSize, imageSize)
 
         visible: false
         showPresenceIndicator: false