VideoCommon: Make API_TYPE an enum class

Allows for forward declarations in most places, which prevents dumping
unrelated VideoCommon.h contents directly into headers.

Source/Core/VideoCommon/TextureConversionShader.h

@@ -5,11 +5,12 @@
 #pragma once
 
 #include "Common/CommonTypes.h"
-#include "VideoCommon/VideoCommon.h"
+
+enum class APIType;
 
 namespace TextureConversionShader
 {
 u16 GetEncodedSampleCount(u32 format);
 
-const char* GenerateEncodingShader(u32 format, API_TYPE ApiType = API_OPENGL);
+const char* GenerateEncodingShader(u32 format, APIType ApiType);
 }