Disable warning C4214: nonstandard extension used: bit field types other than int

This shows up with Visual Studio 2019 and is a non-standard extension that we rely on in this code.

src/video/SDL_blit_slow.c

@@ -294,6 +294,11 @@ typedef union
     } x;
 } FP32;
 
+#ifdef _MSC_VER
+#pragma warning(push)
+#pragma warning(disable:4214)
+#endif
+
 typedef union
 {
     Uint16 u;
@@ -305,6 +310,10 @@ typedef union
     } x;
 } FP16;
 
+#ifdef _MSC_VER
+#pragma warning(pop)
+#endif
+
 static float half_to_float(Uint16 unValue)
 {
     static const FP32 magic = { (254 - 15) << 23 };