Disable faux scrolling when shift is pressed

To make it possible to access the native scrollback buffer in the
alternate screen without having to disable faux scrolling, faux
scrolling is now disabled when the `shift` key is held down.

This should allow alacritty to have the best of both worlds, a native
scrollback buffer in the alternate screen buffer and faux scrolling.

src/input.rs

@@ -483,7 +483,7 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
         if self.ctx.terminal_mode().intersects(mouse_modes) {
             self.mouse_report(code, ElementState::Pressed, modifiers);
         } else if self.ctx.terminal_mode().contains(TermMode::ALT_SCREEN)
-            && faux_scrollback_lines > 0
+            && faux_scrollback_lines > 0 && !modifiers.shift
         {
             // Faux scrolling
             let cmd = code + 1; // 64 + 1 = A, 65 + 1 = B