bug#21835: 25.0.50; cursor height wrong when line-spacing is used

[Eli Zaretskii <eliz@gnu.org>]

> From: David Reitter <david.reitter@gmail.com>
> Date: Thu, 5 Nov 2015 22:12:03 -0500
> Cc: 21835@debbugs.gnu.org
> 
> One could default to the height of the last glyph of the line. Normally, we would expect the size of the cursor to indicate the general height of the text that would be inserted there, and the last glyph is the best guess.

I tried that, and it was worse.  The problem is, the last glyph could
have very small height, at least with some fonts.  Then you have a
very small cursor at the end.  What's more, as you type at EOL, the
cursor height will constantly change.

> Alternatively, use the line height for all cursor positions in that line.  In that case, things would still need to be adjusted for `line-spacing’, and based on what you wrote, I think that’s less preferable.

Indeed.  Our cursor, when it is drawn on a character, shows the
dimensions of that character, both horizontal and vertical.  Losing
that, even only for the vertical dimension, would be a step backward,
I think.

There are also differences in how we draw the hollow cursor (in
non-selected windows) vs the cursor in selected windows.  They should
have the same dimensions, or else it will look like a bug.  The
solution I coded solves this problem as well (some other ideas
didn't).

So once again, I don't know of any better way of solving this.