Update pool type in CreateOffscreenPlainSurface()

[elishacloud]

Pull request has two changes in it:

1. This changes CreateOffscreenPlainSurface() to use D3DPOOL_SCRATCH rather than D3DPOOL_SYSTEMMEM.

First of all, this better simulates how DirectX 8 works. Microsoft documents in CreateOffscreenPlainSurface that:

D3DPOOL_SCRATCH will return a surface that has identical characteristics to a surface created by the DirectX 8.x method CreateImageSurface.

Secondly, this fixes the issue with Forsaken World mentioned here.

2. This change will remove the multi-sample flag rather than the D3DPRESENTFLAG_LOCKABLE_BACKBUFFER flag.

These two flags are not compatible which each other in DirectX 9. However, this is much safer than before. If you remove the multi-sample flag the you could lose is anti-aliasing. However, if you remove the D3DPRESENTFLAG_LOCKABLE_BACKBUFFER flag the game could crash.