Use force inlining of YUV conversion functions

Force inling of the 444->420 conversion functions because their CPU
usage goes up pretty heavily without it when compiling without
optimizations

libobs/util/c99defs.h

@@ -36,6 +36,12 @@
 #define inline __inline
 #endif
 
+#ifdef _MSC_VER
+#define FORCE_INLINE __forceinline
+#else
+#define FORCE_INLINE __attribute__(always_inline)
+#endif
+
 #define EXPORT __declspec(dllexport)
 #else
 #define EXPORT