Adjust the font size according to the number of columns

Additionally, if it is impossible to fit the entire screen buffer on the
current monitor, detect this condition uising the
GetLargestConsoleWindowSize API and make the console window as large as
possible.  This is suboptimal for full-screen console programs (e.g. Far
Manager), but because the scrolling-mode scraper scrapes the screen buffer
rather than the visble window, the smaller-than-desired window frequently
has no noticeable effect.

Revert the use of MARK to freeze the new Windows 10 console.  Use
SELECT_ALL again.

Fixes #61
Fixes #79
Breaks #53 again

src/agent/Agent.cc

@@ -42,6 +42,7 @@
 #include "../shared/WindowsVersion.h"
 #include "../shared/WinptyAssert.h"
 
+#include "ConsoleFont.h"
 #include "ConsoleInput.h"
 #include "NamedPipe.h"
 #include "Scraper.h"
@@ -59,39 +60,48 @@ static BOOL WINAPI consoleCtrlHandler(DWORD dwCtrlType)
     return FALSE;
 }
 
-// In versions of the Windows console before Windows 10, the SelectAll and
-// Mark commands both run quickly, but Mark changes the cursor position read
-// by GetConsoleScreenBufferInfo.  Therefore, use SelectAll to be less
-// intrusive.
+// We can detect the new Windows 10 console by observing the effect of the
+// Mark command.  In older consoles, Mark temporarily moves the cursor to the
+// top-left of the console window.  In the new console, the cursor isn't
+// initially moved.
 //
-// Starting with the new Windows 10 console, the Mark command no longer moves
-// the cursor, and SelectAll uses a lot of CPU time.  Therefore, use Mark.
-//
-// The Windows 10 legacy-mode console behaves the same way as previous console
-// versions, so detect which syscommand to use by testing whether Mark changes
-// the cursor position.
-static void initConsoleFreezeMethod(
+// We might like to use Mark to freeze the console, but we can't, because when
+// the Mark command ends, the console moves the cursor back to its starting
+// point, even if the console application has moved it in the meantime.
+static void detectNewWindows10Console(
         Win32Console &console, Win32ConsoleBuffer &buffer)
 {
-    const ConsoleScreenBufferInfo info = buffer.bufferInfo();
-
-    // Make sure the buffer and window aren't 1x1.  (Is that even possible?)
-    buffer.resizeBuffer(Coord(
-        std::max<int>(2, info.dwSize.X),
-        std::max<int>(2, info.dwSize.Y)));
-    buffer.moveWindow(SmallRect(0, 0, 2, 2));
-    const Coord initialPosition(1, 1);
-    buffer.setCursorPosition(initialPosition);
+    ConsoleScreenBufferInfo info = buffer.bufferInfo();
+
+    // Make sure the window isn't 1x1.
+    if (info.srWindow.Left == info.srWindow.Right &&
+            info.srWindow.Top == info.srWindow.Bottom) {
+        // Make sure the buffer isn't 1x1.
+        if (info.dwSize.X == 1 && info.dwSize.Y == 1) {
+            setSmallFont(buffer.conout(), 80, false);
+            buffer.resizeBuffer(Coord(80, 25));
+            info = buffer.bufferInfo();
+        }
+        buffer.moveWindow(SmallRect(0, 0, 2, 1));
+        info = buffer.bufferInfo();
+        ASSERT(info.srWindow.Right > info.srWindow.Left &&
+            "Could not expand console window from 1x1");
+    }
 
     // Test whether MARK moves the cursor.
+    const Coord initialPosition(info.srWindow.Right, info.srWindow.Bottom);
+    buffer.setCursorPosition(initialPosition);
     ASSERT(!console.frozen());
     console.setFreezeUsesMark(true);
     console.setFrozen(true);
-    const bool useMark = (buffer.cursorPosition() == initialPosition);
+    const bool isNewW10 = (buffer.cursorPosition() == initialPosition);
     console.setFrozen(false);
-    trace("Using %s syscommand to freeze console",
-        useMark ? "MARK" : "SELECT_ALL");
-    console.setFreezeUsesMark(useMark);
+    buffer.setCursorPosition(Coord(0, 0));
+
+    trace("Attempting to detect new Windows 10 console using MARK: %s",
+        isNewW10 ? "detected" : "not detected");
+    console.setFreezeUsesMark(false);
+    console.setNewW10(isNewW10);
 }
 
 static inline WriteBuffer newPacket() {
@@ -141,7 +151,7 @@ Agent::Agent(LPCWSTR controlPipeName,
         m_errorBuffer = Win32ConsoleBuffer::createErrorBuffer();
     }
 
-    initConsoleFreezeMethod(m_console, *primaryBuffer);
+    detectNewWindows10Console(m_console, *primaryBuffer);
 
     m_controlPipe = &connectToControlPipe(controlPipeName);
     m_coninPipe = &createDataServerPipe(false, L"conin");