Specify underlying types for enums that get casted from integers

Otherwise we might get UB if the value we cast is larger than the
max value of the underlying type that the compiled picked for the enum.

I haven't done any extensive check through Dolphin to find cases
of this, I'm just fixing the cases I already know of.

Source/Core/VideoCommon/VideoConfig.h

@@ -24,7 +24,7 @@
 
 constexpr int EFB_SCALE_AUTO_INTEGRAL = 0;
 
-enum class AspectMode
+enum class AspectMode : int
 {
   Auto,
   AnalogWide,
@@ -32,7 +32,7 @@ enum class AspectMode
   Stretch,
 };
 
-enum class StereoMode
+enum class StereoMode : int
 {
   Off,
   SBS,