Graphics modules are sometimes freed before their last usage

[tarek-y-ismail]

How to reproduce:

1. Checkout and build MIRENG-883/support-mouse-keys
2. Run Mir in gdb
3. Exit
4. You should get a SIGSEGV sometime during TouchspotController's destruction

This issue is that TouchspotController doesn't hold on to a reference to the allocator it created its graphics buffer with. So after it creates a buffer and some time before the buffer is destroyed, the module is unloaded. Once we try to destroy TouchspotController and try to destroy the buffer, it tries to call unloaded code, blowing up the whole thing.

The quickest workaround is to keep a reference to the allocator, which itself keeps the module alive. The better solution would be to tie the lifetime of the module to the lifetime of whatever is using it.