text: fix bitmap y offset computation

This commit fixes a bug that would incorrectly baseline bitmap glyphs text if the line
contained another font with a taller line height. The logic for computing the y offset of
the glyph incorrectly assumed that the Glyph.Ascent was particular to the glyph instead of
the line. I've updated it to use a glyph-specific value.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

text/gotext.go

@@ -674,7 +674,7 @@ func (s *shaperImpl) Bitmaps(ops *op.Ops, gs []Glyph) op.CallOp {
 			}
 			off := op.Affine(f32.Affine2D{}.Offset(f32.Point{
 				X: fixedToFloat((g.X - x) - g.Offset.X),
-				Y: fixedToFloat(g.Offset.Y - g.Ascent),
+				Y: fixedToFloat(g.Offset.Y + g.Bounds.Min.Y),
 			})).Push(ops)
 			cl := clip.Rect{Max: imgSize}.Push(ops)