Bugfix #4351

ESC wasn't supported for sequence-based rendering yet.
Solved it with correctly initializing the sequence render with all
render callbacks, including render updates (which it didn't do yet).

Cleanup:

- Bug in ghostwinlay code: the get_mbut() function reads from the window
  struct if a mouse is pressed. However, when you press the mouse in the
  sequencer, which causes a render, this value was hanging because then
  the active window was a render window.

- The new render display options (image window) didn't work for sequence
  render OK. There was a recursion even, because a sequence draw command
  calls a render, which in turn now calls redraws.

source/blender/src/ghostwinlay.c

@@ -450,6 +450,7 @@ void window_set_custom_cursor_ex(Window *win, BCursor *cursor, int useBig) {
 
 void window_make_active(Window *win) {
 	if (win != active_gl_window) {
+		win->lmbut= 0;	/* keeps hanging when mousepressed while other window opened */
 		active_gl_window= win;
 		GHOST_ActivateWindowDrawingContext(win->ghostwin);
 	}