This header doesn't actually make use of MathUtil.h within itself, so
this can be removed. Many other source files used VideoCommon.h as an
indirect include to include MathUtil.h, so these includes can also be
adjusted.
While we're at it, we can also migrate valid inclusions of VideoCommon.h
into cpp files where it can feasibly be done to minimize propagating it
via other headers.
It appears that some older drivers do not support
CreateSwapChainForHwnd, resulting in DXGI_ERROR_INVALID_CALL. For these
cases, fall back to the base CreateSwapChain() from DXGI 1.0.
In theory this should also let us run on Win7 without the platform
update, but in reality we require the newer shader compiler so this
probably won't work regardless. Also any hardware of this vintage is
unlikely to run Dolphin well.
