Patch to vertically center line content when using line-spacing variable

[Jesse Medeiros]

When you set the line-spacing variable, Emacs only adds margin to the
bottom of the line, which always annoyed me. So I decided to work on
this and wrote a patch to vertically center the line when setting the
line-spacing variable. I created a new variable called
line-spacing-vertical-center, and the centering will only apply when
this variable is set to non-nil. Could this be merged upstream? Here's
the link for the patch:

https://github.com/sollidsnake/emacs/commit/aff5c59d10001e1161884ef3d0725eda86fa3ea0

This is my first attempt to contribute to Emacs, and I'm not very
familiar neither with C nor with Emacs core, so please let me know if
I'm doing something wrong.


Jesse Medeiros