Initial test of multi-macOS builds

This is an initial test of building Barrier for multiple macOS versions.

For:

- Big Sur
- Moja\ve
- Catalina

I think there's an indentation issue somewhere, but VS Code isn't
'seeing' it... let's hope this works.

If it does, we can proceed to merge.

Signed-off-by: Dom Rodriguez <shymega@shymega.org.uk>
(cherry picked from commit 59d076988f)

azure-pipelines.yml

@@ -81,11 +81,18 @@ jobs:
 
 - job: MacBuild
   displayName: Mac Build
-  pool:
-    vmImage: 'macOS-10.14'
   strategy:
     matrix:
-      Release:
+        big-sur-Release:
+            imageName: "macOS-11"
+            B_BUILD_TYPE: Release
+            BARRIER_VERSION_STAGE: Release
+        catalina-Release:
+            imageName: "macOS-10.15"
+            B_BUILD_TYPE: Release
+            BARRIER_VERSION_STAGE: Release
+        mojave-Release:
+            imageName: "macOS-10.14"
         B_BUILD_TYPE: Release
         BARRIER_VERSION_STAGE: Release
   variables: