bug#12245: 24.2.50; Image height scaled to fill whole line

[Ivan Andrus <darthandrus@gmail.com>]

A fairly recent change (in the past few weeks) changed preview-latex in an
interesting way.  I would bisect it, but I have never been able to get
`bzr bisect` to work.

What happens, is that the image heights are stretched to fill the full
height of the current line.  If the image is larger, however, it is not
shrunk.  This leads to problems reading the short images like those
produced from $x$, when they are on a line with a tall equation like a
sum.  

The problem can be see without preview-latex.  Start from `emacs -Q` and
evaluate the code below.  Notice that both images are the same height
(larger than a normal line--this works for me with the default font
size).  Then delete the second image and notice that the first image
shrinks.  In fact the image should be even smaller.

    (progn (insert (propertize "1" 'display
                               (create-image "/* XPM */
    static char * arrow_right[] = {
    \"12 5 2 1\",
    \".	c grey60\",
    \" 	c None\",
    \".           \",
    \"..          \",
    \"...         \",
    \"....        \",
    \".....       \",
    \"......      \",
    \".......     \",
    \"........    \",
    \".........   \",
    \".........   \",
    \"........    \",
    \".......     \",
    \"......      \",
    \".....       \",
    \"....        \",
    \"...         \",
    \"..          \",
    \".           \"};"
                                             'xpm t)))
           (insert (propertize "1" 'display
                               (create-image "/* XPM */
    static char * arrow_right[] = {
    \"12 24 2 1\",
    \".	c grey60\",
    \" 	c None\",
    \".           \",
    \"..          \",
    \"...         \",
    \"....        \",
    \".....       \",
    \"......      \",
    \".......     \",
    \"........    \",
    \".........   \",
    \".........   \",
    \".........   \",
    \".........   \",
    \".........   \",
    \".........   \",
    \".........   \",
    \".........   \",
    \"........    \",
    \".......     \",
    \"......      \",
    \".....       \",
    \"....        \",
    \"...         \",
    \"..          \",
    \".           \"};"
                                             'xpm t))))

                                         


In GNU Emacs 24.2.50.2 (i386-apple-darwin10.8.0, NS apple-appkit-1038.36)
of 2012-08-20 on oroszlan.local
Bzr revision: 109700 jlf@foxtail.org-20120820160851-cooolx696cb26xc0
Windowing system distributor `Apple', version 10.3.1038
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