LineSpacing font metric does not match with Windows

[handerss-spotfire]

(Using libgdiplus built from main with Pango as backend)

For some fonts the vertical metric "line spacing" does not match between libgdiplus and GDI+. One example is Roboto Regular (which can be retrieved here: https://fonts.google.com/specimen/Roboto).

On Windows GDI+ reports the following metrics:
Ascent: 1946
Descent: 512
LineSpacing: 2458

On Linux libgdiplus reports the following metrics (for the same .ttf file):
Ascent: 1946
Descent: 512
LineSpacing: 2400

The reason for this seems to be that line spacing is set using ascender + descender + gap before ascender and descender are fully determined. Setting it afterwards results in the expected metrics:

diff --git a/src/font.c b/src/font.c
index 9a4ac3a..931ba4d 100644
--- a/src/font.c
+++ b/src/font.c
@@ -914,8 +914,6 @@ gdip_get_fontfamily_details_from_harfbuzz (GpFontFamily *family, hb_font_t *font
        hb_font_set_scale (subfont, family->height, family->height);
        hb_font_get_h_extents (subfont, &font_extents);

-       family->linespacing = font_extents.line_gap + font_extents.ascender - font_extents.descender;
-
        if (hb_ot_metrics_get_position (subfont, HB_OT_METRICS_TAG_HORIZONTAL_CLIPPING_ASCENT, &position)) {
                family->cellascent = position;
        } else {
@@ -927,6 +925,8 @@ gdip_get_fontfamily_details_from_harfbuzz (GpFontFamily *family, hb_font_t *font
                family->celldescent = -font_extents.descender;
        }

+       family->linespacing = font_extents.line_gap + family->cellascent + family->celldescent;
+
        hb_font_destroy (subfont);
 }
 #endif