The Vulkan swapchain is unaffected by this, but we don't create an
"internal" frontbuffer in D3D9SwapChainEx if this option is set. This
breaks GetFrontBufferData (which returns backbuffer data if the option
is enabled), but it disables front/backbuffer flipping.

Most windows drivers apparently always use the same backbuffer for all
frames in windowed mode. At least one game (ZUSI 3) seems to rely on
this behavior, and only redraws dirty regions for each frame instead of
redrawing everything. With buffer flips, this leads to flickering. When
enabling this new noExplicitFrontBuffer option, the flickering
disappears.

Fixes flickering when parts of the screen are not redrawn in a frame.

Closes: doitsujin#1368