misc: fix PushButton renderer for Linux

Gitlab: #140

Change-Id: I980126d333ab33c9fbc9f2984d115d79082b0dd3

src/commoncomponents/PushButton.qml

@@ -150,6 +150,9 @@ AbstractButton {
                             return JamiTheme.transparentColor
                     }
                 }
+                // Mipmap does not render correctly on linux
+                mipmap: false
+                smooth: true
             }
         }