Fix various Read/WriteConsoleOutput bugs (#17567)

Split off from #17510:
* `Viewport::Clamp` used `std::clamp` to calculate the intersection
  between two rectangles. That works for exclusive rectangles,
  because `.left == .right` indicates an empty rectangle.
  But `Viewport` is an inclusive one, and so `.left == .right` is
  non-empty. For instance, if the to-be-clamped rect is fully
  outside the bounding rect, the result is a 1x1 viewport.
  In effect this meant that `Viewport::Clamp` never clamped so far.
* The `targetArea < targetBuffer.size()` check is the wrong way around.
  It should be `targetArea > targetBuffer.size()`.
* The `sourceSize` and `targetSize` checks are incorrect, because the
  rectangles may be non-empty but outside the valid bounding rect.
* If these sizes were empty, we'd return the requested rectangle which
  is a regression since conhost v1 and violates the API contract.
* The `sourceRect` emptiness check is incorrect, because the clamping
  logic before it doesn't actually clamp to the bounding rect.
* The entire clamping and iteration logic is just overall too complex.

tools/OpenConsole.psm1

@@ -197,7 +197,7 @@ function Invoke-OpenConsoleTests()
     {
         $OpenConsolePlatform = 'Win32'
     }
-    $OpenConsolePath = "$env:OpenConsoleroot\bin\$OpenConsolePlatform\$Configuration\OpenConsole.exe"
+    $OpenConsolePath = "$root\bin\$OpenConsolePlatform\$Configuration\OpenConsole.exe"
     $TaefExePath = "$root\packages\Microsoft.Taef.10.60.210621002\build\Binaries\$Platform\te.exe"
     $BinDir = "$root\bin\$OpenConsolePlatform\$Configuration"