Rename PostCSS files to .pcss (#9013)

* Rename PostCSS files to `.pcss`

* Make Stylelint happy

* Delint

* Rename new files too

* delint

* Fix bad comment placement

res/css/structures/_BackdropPanel.pcss

@@ -0,0 +1,38 @@
+/*
+Copyright 2021 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+.mx_BackdropPanel {
+    position: absolute;
+    left: 0;
+    top: 0;
+    height: 100vh;
+    width: 100%;
+    overflow: hidden;
+    filter: blur(var(--lp-background-blur));
+    // Force a new layer for the backdropPanel so it's better hardware supported
+    transform: translateZ(0);
+}
+
+.mx_BackdropPanel--image {
+    position: absolute;
+    top: 0;
+    left: 0;
+    min-height: 100%;
+    z-index: 0;
+    pointer-events: none;
+    overflow: hidden;
+    user-select: none;
+}