Allow usage of the new Condition Variable code with Critical Sections

Vista and later provide the SleepConditionVariableCS() function for this.

Since SDL_syscond_srw.c doesn't require SRW locks anymore, rename it to
SDL_syscond_cv.c which better reflects the implementation of condition
variables rather than the implementation of mutexes.

Fixes #4051.

src/thread/windows/SDL_sysmutex_c.h

@@ -66,4 +66,9 @@ typedef struct SDL_mutex_srw
     DWORD owner;
 } SDL_mutex_srw;
 
+typedef struct SDL_mutex_cs
+{
+    CRITICAL_SECTION cs;
+} SDL_mutex_cs;
+
 /* vi: set ts=4 sw=4 expandtab: */