Work around Vindictus constant buffer breakage

[doitsujin]

I hate everything about this, The game has some 432-byte constant buffer that it constantly DISCARDs but doesn't really bother writing to, yet it accesses the uninitialized data in the shader.

Before the memory rework, this was sort-of fine because per-buffer allocations were more or less set in stone, we'd grab some memory, clear it and then just reuse those regions, so any data not written at all would remain zeroed. The new allocator however allocates fresh memory on every MAP_WRITE_DISCARD, which means the memory we return to the app might have been used for a different purpose before and can contain stale data.

This workaround essentially zeroes all mapped memory allocations when they are freed. While this mostly happens on a worker thread, it is very CPU-intensive and slow, but I don't see any better ways to fix this game.

Fixes #4405.