st/nine: Improve pDestRect handling

st/nine: Improve pDestRect handling

pSourceRect and pDestRect allows to:
A) display a crop of the backbuffer
B) display the content in a subregion (after an offset)
C) resize the content before displaying

Before this patch, only features A and B were supported.
This patch adds C, but breaks A, which current support
relied on C not being implemented.

I think C is more important than A, and A can be added later.

Signed-off-by: Axel Davy <davyaxel0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5015>