Codechange: Replace pointer to Sprite array with reference to SpriteCollection. (#11580)

Add `SpriteLoader::SpriteCollection` type which is an array of `SpriteLoad::Sprite`.

This removes the ambiguity of what `SpriteLoader::Sprite *` is pointing to,
and cleans up mismatches using both dereference -> and array access [] for the
same object.

src/video/opengl.h

@@ -109,7 +109,7 @@ public:
 
 	bool Is32BppSupported() override { return true; }
 	uint GetSpriteAlignment() override { return 1u << (ZOOM_LVL_END - 1); }
-	Sprite *Encode(const SpriteLoader::Sprite *sprite, AllocatorProc *allocator) override;
+	Sprite *Encode(const SpriteLoader::SpriteCollection &sprite, AllocatorProc *allocator) override;
 };