Current DirectX capturer cannot capture multiple video cards or monitors. But
according to DXGI desktop duplication sample (https://goo.gl/An0L9l), we can
capture multiple video cards and monitors by duplicating them one by one. So
instead of one IDXGIOutputDuplication instance, this change creates an
IDXGIOutputDuplication instance for each monitor, and merge the output into
one DesktopFrame.
Several other changes are also included,
1. Add supports to DXGI_OUTDUPL_DESC.DesktopImageInSystemMemory. When this flag
is true, we won't copy its content to staging buffer.
2. Capture one monitor instead of entire screen.
Above changes make the logic complex. But with some refactor work, the logic is
not disordered. Please refer to the doc @ https://goo.gl/hU1ifG.
BUG=314516
Review-Url: https://codereview.webrtc.org/2099123002
Cr-Commit-Position: refs/heads/master@{#13684}
