CMake: Rquire CMake 3.16

The recent graphics capture changes require CMake 3.16 or above to
function on Windows, so add a requirement check for 3.16 if on Windows.

CMakeLists.txt

@@ -10,6 +10,7 @@ project(obs-studio)
 option(BUILD_CAPTIONS "Build captions" FALSE)
 
 if(WIN32)
+	cmake_minimum_required(VERSION 3.16)
 	if (QTDIR OR DEFINED ENV{QTDIR} OR DEFINED ENV{QTDIR32} OR DEFINED ENV{QTDIR64})
 		# Qt path set by user or env var
 	else()