Fix/silence mac build warnings

As per Xcode 12.4 you can at least do a warning-free incremental build
with these changes. There are still plenty of warnings in a full build
(mostly from thirdparty components) but with these changes you can at
least develop on mac and see if/when you've added any new warnings when
doing incremental builds.

src/CMakeLists.txt

@@ -117,6 +117,7 @@ if(WIN32)
         ${SPACEWARE_LIBRARIES})
 elseif(APPLE)
     add_compile_options(
+        -DGL_SILENCE_DEPRECATION
         -fobjc-arc)
 
     list(APPEND platform_SOURCES

src/platform/guimac.mm

@@ -286,7 +286,8 @@ public:
     }
 
     void PopUp() override {
-        [NSMenu popUpContextMenu:nsMenu withEvent:[NSApp currentEvent] forView:nil];
+        NSEvent* event = [NSApp currentEvent];
+        [NSMenu popUpContextMenu:nsMenu withEvent:event forView:event.window.contentView];
     }
 
     void Clear() override {