Blitter: Null blitter always requires no zoom levels

src/blitter/null.hpp

@@ -18,6 +18,7 @@ public:
 	Blitter_Null()
 	{
 		this->SetScreenDepth(0);
+		this->SetNoSpriteDataRequired(true);
 	}
 
 	void Draw(Blitter::BlitterParams *bp, BlitterMode mode, ZoomLevel zoom) override {};

src/spritecache.cpp

@@ -605,6 +605,7 @@ static void *ReadSprite(const SpriteCache *sc, SpriteID id, SpriteType sprite_ty
 	} else {
 		zoom_levels = UINT8_MAX;
 	}
+	if (encoder->NoSpriteDataRequired()) zoom_levels = 0;
 
 	SpriteFile &file = *sc->file;
 	size_t file_pos = sc->file_pos;

src/spriteloader/spriteloader.hpp

@@ -84,6 +84,7 @@ public:
 class SpriteEncoder {
 	bool supports_missing_zoom_levels = false;
 	bool supports_32bpp = false;
+	bool no_data_required = false;
 
 protected:
 	inline void SetSupportsMissingZoomLevels(bool supported)
@@ -96,6 +97,11 @@ protected:
 		this->supports_32bpp = supported;
 	}
 
+	inline void SetNoSpriteDataRequired(bool not_required)
+	{
+		this->no_data_required = not_required;
+	}
+
 public:
 
 	virtual ~SpriteEncoder() = default;
@@ -105,6 +111,11 @@ public:
 		return this->supports_missing_zoom_levels;
 	}
 
+	inline bool NoSpriteDataRequired() const
+	{
+		return this->no_data_required;
+	}
+
 	/**
 	 * Can the sprite encoder make use of RGBA sprites?
 	 */