Revert "Preallocate memory for image/file presence caches"

This reverts commit a3ef80853c.

Vector reallocation cost is logarithmic. Assuming that a vector
starts at a size of one and doubles its size in each allocation, the
result is 11 allocations - completely negligible compared with, you
know, loading 20 000 images from disk.

src/image.cpp

@@ -108,7 +108,6 @@ public:
 	cache_type()
 		: content_()
 	{
-		content_.reserve(20000);
 	}
 
 	cache_item<T>& get_element(int index)