TextureCache: load all mipmap levels from custom textures

This drops the "feature" to load level 0 from the custom texture
and all other levels from the native one if the size matches.
But in my opinion, when a custom texture only provide one level,
no more should be used at all.

Source/Core/VideoCommon/TextureCacheBase.h

@@ -52,6 +52,7 @@ public:
 		u64 hash;
 		u32 format;
 		bool is_efb_copy;
+		bool is_custom_tex;
 
 		unsigned int native_width, native_height; // Texture dimensions from the GameCube's point of view
 		unsigned int native_levels;