Engine: D3D use GetBackBuffer instead of GetFrontBufferData

Uses the much faster and allegedly reliable GetBackBuffer instead of GetFrontBufferData inside the D3D version of GetCopyOfScreenIntoBitmap()

I wonder if the D3DPRESENTFLAG_LOCKABLE_BACKBUFFER flag might cause issues on stuff like video reproduction, so some testing may be in order.

Note: it would need some tinkering to make bitdepths != 32 to work, but since that was broken to begin with this should be an outright improvement.