Rework backdrop to draw one image with two different level of blur

res/css/structures/_MatrixChat.scss

@@ -39,10 +39,10 @@ limitations under the License.
     position: absolute;
     top: 0;
     left: 0;
-    width: 100%;
     min-height: 100%;
     z-index: 0;
     pointer-events: none;
+    overflow: hidden;
 }
 
 .mx_MatrixToolbar {
@@ -76,7 +76,7 @@ limitations under the License.
 }
 
 /* not the left panel, and not the resize handle, so the roomview/groupview/... */
-.mx_MatrixChat > :not(.mx_LeftPanel):not(.mx_SpacePanel):not(.mx_ResizeHandle) {
+.mx_MatrixChat > :not(.mx_LeftPanel):not(.mx_SpacePanel):not(.mx_ResizeHandle):not(canvas) {
     background-color: $primary-bg-color;
 
     flex: 1 1 0;