Fix vertical cursor motion across tall text or small images

Fix vertical cursor motion across tall text or small images

'line-move-partial' should in general leave it to the display
engine to scroll or recenter the window due to vertical motion of
the cursor.  The only purpose of this function is to produce
vscroll suitable for scrolling across large (relatively to the
window's height) images, where moving by display lines is not
appropriate.

* src/xdisp.c (Fdisplay__line_is_continued_p): New primitive.

* lisp/simple.el (line-move-partial): Call
'display--line-is-continued-p' to decide whether to leave it to
redisplay to scroll the window as appropriate.  (Bug#48170)