Kernel: More signal handling improvements.

Finally fixed the weird flaky crashing when resizing Terminal windows.
It was because we were dispatching a signal to "current" from the scheduler.
Yet another thing I dislike about even having a "current" process while
we're in the scheduler. Not sure yet how to fix this.

Let the signal handler's kernel stack be a kmalloc() allocation for now.
Once we can do allocation of consecutive physical pages in the supervisor
memory region, we can use that for all types of kernel stacks.

SharedGraphics/Painter.cpp

@@ -75,6 +75,8 @@ void Painter::fill_rect(const Rect& a_rect, Color color)
     if (rect.is_empty())
         return;
 
+    ASSERT(m_target->rect().contains(rect));
+
     RGBA32* dst = m_target->scanline(rect.top()) + rect.left();
     const unsigned dst_skip = m_target->width();