Cache the block which most recently successfully allocated something

Otherwise, once the most recently allocated block was full, all allocations would need to scan the whole collection of blocks to find free space, which could take a while if lots of blocks had already been allocated but contained free space due to things being freed.

src/vsg/core/IntrusiveAllocator.cpp

@@ -745,7 +745,11 @@ void* IntrusiveAllocator::MemoryBlocks::allocate(std::size_t size)
         if (block != memoryBlockWithSpace)
         {
             auto ptr = block->allocate(size);
-            if (ptr) return ptr;
+            if (ptr)
+            {
+                memoryBlockWithSpace = block;
+                return ptr;
+            }
         }
     }