render: add font_baseline() - calculates the y-coordinate for the baseline

The old baseline calculation was copy-pasted to a couple of places,
and also assumed that the font's height was equal to ascent+descent.

While this is typically true, it isn't necessarily so.

Now, we assume that height >= ascent+descent, and then position the
baseline in "center" (but adjusted for the descent).

render.c

@@ -80,6 +80,25 @@ pixman_color_dim_for_search(pixman_color_t *color)
     color->blue /= 3;
 }
 
+static inline int
+font_baseline(const struct terminal *term)
+{
+    assert(term->fextents.ascent >= 0);
+    assert(term->fextents.descent >= 0);
+
+    int diff = term->fextents.height - (term->fextents.ascent + term->fextents.descent);
+    assert(diff >= 0);
+
+#if 0
+    LOG_INFO("height=%d, ascent=%d, descent=%d, diff=%d",
+             term->fextents.height,
+             term->fextents.ascent, term->fextents.descent,
+             diff);
+#endif
+
+    return term->fextents.height - diff / 2 - term->fextents.descent;
+}
+
 static void
 draw_bar(const struct terminal *term, pixman_image_t *pix,
          const pixman_color_t *color, int x, int y)