Make ctrl+shift+scroll toggle acrylic on/off at extrema (microsoft#4853)

If UseAcrylic is disabled, CTRL+SHIFT+SCROLL would enable it, without
having to change the setting in profile.json manually.

1. Set "useAcrylic" to false for the any profile in profile.json
2. Open terminal window for that profile.
3. CTRL+SHIFT+MouseScroll
   Acrylic background opacity should change according to mouse scroll

## PR Checklist
* [x] CLA signed.
* [x] Tested manually
* [x] Updated documentation

Closes microsoft#661

doc/user-docs/index.md

@@ -85,6 +85,6 @@ For an introduction to the various settings, see [Using Json Settings](UsingJson
     (ref [https://twitter.com/r_keith_hill/status/1142871145852440576](https://twitter.com/r_keith_hill/status/1142871145852440576))
 
 2. Terminal zoom can be changed by holding <kbd>Ctrl</kbd> and scrolling with mouse.
-3. If `useAcrylic` is enabled in profiles.json, background opacity can be changed by holding <kbd>Ctrl</kbd>+<kbd>Shift</kbd> and scrolling with mouse. Note that acrylic transparency is limited by the OS only to focused windows.
+3. Background opacity can be changed by holding <kbd>Ctrl</kbd>+<kbd>Shift</kbd> and scrolling with mouse. Note that acrylic transparency is limited by the OS only to focused windows.
 4. Open Windows Terminal in current directory by typing `wt -d .` in the address bar.
 5. Please add more Tips and Tricks.

src/cascadia/TerminalControl/TermControl.cpp

@@ -1095,9 +1095,24 @@ namespace winrt::Microsoft::Terminal::TerminalControl::implementation
             {
                 auto acrylicBrush = RootGrid().Background().as<Media::AcrylicBrush>();
                 acrylicBrush.TintOpacity(acrylicBrush.TintOpacity() + effectiveDelta);
+                if (acrylicBrush.TintOpacity() == 1.0)
+                {
+                    _settings.UseAcrylic(false);
+                    _InitializeBackgroundBrush();
+                    uint32_t bg = _settings.DefaultBackground();
+                    _BackgroundColorChanged(bg);
+                }
             }
             CATCH_LOG();
         }
+        else if (mouseDelta < 0)
+        {
+            _settings.UseAcrylic(true);
+
+            //Setting initial opacity set to 1 to ensure smooth transition to acrylic during mouse scroll
+            _settings.TintOpacity(1.0);
+            _InitializeBackgroundBrush();
+        }
     }
 
     // Method Description: