Flag to defer screen repaints

Perhaps this exists already and I have missed it. I want a flag that suppresses all screen repaints while a program makes use of calls like CLLCD and PIXEL.

To see why, load up my test program from #555, then observe that:

1. The program appears to do nothing on the actual R47 hardware. I suspect the HAL is refreshing the screen in the background, continually overwriting my black screen with the stack, status bar, menus, etc.

2. Programmatically calling PAUSE immediately prior to the RTN is ineffective for holding the black screen in place long enough to screenshot it before it goes away.

3. SNAP appears to cause a repaint and then takes the snapshot of the rebuilt stack display, not the drawn picture.

All this is in support of my Mandelbrot set program, where I want to wait for a keypress before restoring the normal stack view, possibly save a BMP via SNAP, either via the keyboard or programmatically.

This flag should auto-reset on END to prevent a program from locking the screen up indefinitely, requiring a hardware reset.