Fix memory leak in game lobby color dropdown textures

[11EJDE11]

Refactor XNAClientColorDropDown to create color textures once at initialization and swap between them and DisabledItemTexture, rather than recreating textures every time the map changes. This prevents a memory leak caused by repeatedly creating new texture objects without disposing old ones.

To test the memory leak fix: Go to Skirmish lobby Choose first map Hold down arrow to scroll through all maps Check memory usage

With 8 players and 8 colors, 64 textures created each time and not disposed.