Revert "Fixed an occasional crash resulting from multi-thread access of the image cache"

This reverts commit 6d0b7c8424. Turns out there's
a better way to fix this problem without using mutexes, which have a noticeable
performance hit (5.2% of the execution time of game_display::draw_invalidated()
according to @jyrkive).

src/image.cpp

@@ -43,7 +43,6 @@
 #include <boost/algorithm/string.hpp>
 #include <boost/functional/hash_fwd.hpp>
 
-#include <mutex>
 #include <set>
 
 static lg::log_domain log_display("display");
@@ -108,30 +107,23 @@ class cache_type
 public:
 	cache_type()
 		: content_()
-		, cache_lock_()
 	{
 	}
 
 	cache_item<T>& get_element(int index)
 	{
-		std::lock_guard<std::mutex> lock(cache_lock_);
-
-		if(static_cast<unsigned>(index) >= content_.size()) {
+		if(static_cast<unsigned>(index) >= content_.size())
 			content_.resize(index + 1);
-		}
-
 		return content_[index];
 	}
 
 	void flush()
 	{
-		std::lock_guard<std::mutex> lock(cache_lock_);
 		content_.clear();
 	}
 
 private:
 	std::vector<cache_item<T>> content_;
-	std::mutex cache_lock_;
 };
 
 template<typename T>