Mutex-protect the ring buffers (as we do in the *nix builds)

This fixes an observed crash in sexyz for Windows. The crash was in RingBufWrite() where 'remain' was set to -1 (4.2B) apparently because pHead and/or pTail changed (to pTail=pHead) during the invocation of the RINGBUF_FILL_LEVEL() macro, because it wasn't thread-safe.

Amazing this problem never came up before.

src/sbbs3/sexyz.vcxproj

@@ -66,7 +66,7 @@
     <ClCompile>
       <Optimization>Disabled</Optimization>
       <AdditionalIncludeDirectories>%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
-      <PreprocessorDefinitions>_DEBUG;WIN32;_CONSOLE;SBBS_EXPORTS;RINGBUF_SEM;RINGBUF_EVENT;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>_DEBUG;WIN32;_CONSOLE;SBBS_EXPORTS;RINGBUF_SEM;RINGBUF_EVENT;RINGBUF_MUTEX;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <BasicRuntimeChecks>EnableFastChecks</BasicRuntimeChecks>
       <RuntimeLibrary>MultiThreadedDebug</RuntimeLibrary>
       <PrecompiledHeaderOutputFile>.\msvc.win32.debug\sexyz/sexyz.pch</PrecompiledHeaderOutputFile>
@@ -110,7 +110,7 @@
       <Optimization>MaxSpeed</Optimization>
       <InlineFunctionExpansion>OnlyExplicitInline</InlineFunctionExpansion>
       <AdditionalIncludeDirectories>%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
-      <PreprocessorDefinitions>NDEBUG;WIN32;_CONSOLE;SBBS_EXPORTS;RINGBUF_SEM;RINGBUF_EVENT;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>NDEBUG;WIN32;_CONSOLE;SBBS_EXPORTS;RINGBUF_SEM;RINGBUF_EVENT;RINGBUF_MUTEX;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <StringPooling>true</StringPooling>
       <RuntimeLibrary>MultiThreadedDLL</RuntimeLibrary>
       <FunctionLevelLinking>true</FunctionLevelLinking>