Add new overlay-backdrop() mixin

- Update modal and offcanvas backdrops to use the mixin (placeholder wouldn't work with params I believe)
- Plus, modals and offcanvases should be able to be used together, so separate values for z-index (already implemented) are reflected properly here.
- Offcanvas still extends modals variables by default, save for z-index

scss/_mixins.scss

@@ -22,6 +22,7 @@
 
 // Components
 @import "mixins/alert";
+@import "mixins/backdrop";
 @import "mixins/buttons";
 @import "mixins/caret";
 @import "mixins/pagination";

scss/_modal.scss

@@ -85,17 +85,7 @@
 
 // Modal background
 .modal-backdrop {
-  position: fixed;
-  top: 0;
-  left: 0;
-  z-index: $zindex-modal-backdrop;
-  width: 100vw;
-  height: 100vh;
-  background-color: $modal-backdrop-bg;
-
-  // Fade for backdrop
-  &.fade { opacity: 0; }
-  &.show { opacity: $modal-backdrop-opacity; }
+  @include overlay-backdrop($zindex-modal-backdrop, $modal-backdrop-bg, $modal-backdrop-opacity);
 }
 
 // Modal header

scss/_offcanvas.scss

@@ -14,6 +14,10 @@
   @include transition(transform $offcanvas-transition-duration ease-in-out);
 }
 
+.offcanvas-backdrop {
+  @include overlay-backdrop($zindex-offcanvas, $offcanvas-backdrop-bg, $offcanvas-backdrop-opacity);
+}
+
 .offcanvas-header {
   display: flex;
   align-items: center;

scss/_variables.scss

@@ -1453,6 +1453,8 @@ $offcanvas-title-line-height:       $modal-title-line-height !default;
 $offcanvas-bg-color:                $modal-content-bg !default;
 $offcanvas-color:                   $modal-content-color !default;
 $offcanvas-box-shadow:              $modal-content-box-shadow-xs !default;
+$offcanvas-backdrop-bg:             $modal-backdrop-bg !default;
+$offcanvas-backdrop-opacity:        $modal-backdrop-opacity !default;
 // scss-docs-end offcanvas-variables
 
 // Code

scss/mixins/_backdrop.scss

@@ -0,0 +1,14 @@
+// Shared between modals and offcanvases
+@mixin overlay-backdrop($zindex, $backdrop-bg, $backdrop-opacity) {
+  position: fixed;
+  top: 0;
+  left: 0;
+  z-index: $zindex-modal-backdrop;
+  width: 100vw;
+  height: 100vh;
+  background-color: $modal-backdrop-bg;
+
+  // Fade for backdrop
+  &.fade { opacity: 0; }
+  &.show { opacity: $modal-backdrop-opacity; }
+}