Warnings: Add missing field initialisers warning

extras/Build/CMake/JUCEHelperTargets.cmake

@@ -65,6 +65,7 @@ elseif((CMAKE_CXX_COMPILER_ID STREQUAL "Clang") OR (CMAKE_CXX_COMPILER_ID STREQU
         -Wpedantic
         -Wdeprecated
         -Wfloat-equal
+        -Wmissing-field-initializers
         $<$<OR:$<COMPILE_LANGUAGE:CXX>,$<COMPILE_LANGUAGE:OBJCXX>>:
             -Wzero-as-null-pointer-constant
             -Wunused-private-field
@@ -94,6 +95,7 @@ elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
         -Wno-strict-overflow
         -Wshadow
         -Wfloat-equal
+        -Wmissing-field-initializers
         $<$<COMPILE_LANGUAGE:CXX>:
             -Woverloaded-virtual
             -Wreorder

extras/Projucer/Source/ProjectSaving/jucer_ProjectExporter.h

@@ -339,7 +339,8 @@ public:
                     "-Wswitch-enum",
                     "-Wuninitialized",
                     "-Wunreachable-code",
-                    "-Wunused-parameter"
+                    "-Wunused-parameter",
+                    "-Wmissing-field-initializers"
                 };
 
                 result.cpp = {

modules/juce_core/native/juce_Threads_mac.mm

@@ -128,10 +128,10 @@ bool Thread::createNativeThread (Priority priority)
     struct ThreadData
     {
         Thread& thread;
-        std::promise<bool> started;
+        std::promise<bool> started{};
     };
 
-    ThreadData threadData { *this };
+    ThreadData threadData { *this, {} };
 
     threadId = threadHandle = makeThreadHandle (attribute, &threadData, [] (void* userData) -> void*
     {