Avoid making new allocation to clear the screen. (#420)

Previously, a new 2D vector was allocated for every new frame. This
caused a lot of temporary allocation to be made.

This patch modify "Screen::Clear" so that it do make a new allocation,
but clear the existing one instead.

Bug:https://github.com/ArthurSonzogni/FTXUI/issues/290#issuecomment-1153327251