Add Yield function to global namespace

In order to develop a threadsafe texture compression function, we will need
to preempt threads in order to not kill performance while we initialzie everything...

Core/include/TexComp.h

@@ -121,4 +121,8 @@ typedef void (* CompressionFuncWithStats)(
 // compressed image and a raw image.
 extern double ComputePSNR(const CompressedImage &ci, const ImageFile &file);
 
+// This is a multi-platform yield function that preempts the current thread
+// based on the threading library that we're using.
+extern void YieldThread();
+
 #endif //_TEX_COMP_H_

Core/src/TexComp.cpp

@@ -49,6 +49,7 @@
 #include <assert.h>
 
 #include "BC7Compressor.h"
+#include "Thread.h"
 #include "WorkerQueue.h"
 #include "ThreadGroup.h"
 
@@ -322,3 +323,7 @@ bool CompressImageData(
 
   return true;
 }
+
+void YieldThread() {
+	TCThread::Yield();
+}