While the _s functions are for security as they avoid exceeding the
supplied buffer, in our case they are not really required as we are
allocating a buffer large enough to store the entire result.
Fixes#171
ID3D11DeviceContext_Map by default will force a CPU sync if the prior call to
CopyResource has not completed, this change defers the mapping and sets the
D3D11_MAP_FLAG_DO_NOT_WAIT when attempting to map the texture allowing the
capture to continue without incurring an expensive CPU/GPU sync.
A new tuneable has also been added
* dxgi:maxTextures
