Fix multiline text line spacing to account for font descenders

The line height calculation for multiline text used getbbox('A') which
only measures the capital letter A height, missing descenders from
characters like g, j, p, q, y. This caused overlapping lines when
multiline text contained lowercase letters with descenders.

Use 'Aj' instead so the bbox captures both ascent (from A) and
descent (from j), giving correct line spacing for all text.

Fixes #1646

Author: armorbreak001

src/PIL/ImageText.py

@@ -353,7 +353,7 @@ def _split(
         fontmode = self._get_fontmode()
         line_spacing = (
             self.font.getbbox(
-                "A",
+                "Aj",
                 fontmode,
                 None,
                 self.features,