Fix text view's rendering on macOS (AppKit)

Locking on a child view while inside the parent's -drawRect:
was always looking horrendously (alas, this is how ROOT text
rendering is implemented in TGTextView et al.). Finally,
with macOS 10.14 released, after all these years of horrors, it
stopped  working at all. While it's still quite crappy that
TGTextView tries to render text in its child window - TGViewFrame
- it does not mean literally we physically have to do this. A
little cheat and we render in the correct view instead.