Delay \fay baseline shear until last text layout step

The current code calculates the effect of \fay on advances
early in retrieve_glyph, only to ignore it until reorder_text,
which then partially undoes these calculations.

Instead, just calculate it once, when all the necessary
properties have been fully determined.

The earliest point to do this would be within reorder_text,
but to simplify future logic changes, delay this until all
other text layout steps are done. This matches VSFilter, which
applies shear after alignment. This would also have neutralized
the bug that the previous commit fixed, because glyphs[0].pos.y
really is always 0 until shear is applied. This also opens
the door for another commit to skip glyphs that are invisible
but affect layout.

In addition to simplifying code and calculations, this commit fixes
a bug: the current code for undoing shear assumes constant scale_x/y
from the last \fay change or line break point to the next such point,
producing strange, wrong positions when scale_x/y do change.

This fixes https://github.com/libass/libass/issues/465.

1 files changed, 5 insertions, 0 deletions

diff --git a/libass/ass_shaper.c b/libass/ass_shaper.c
index 4665e7a..ffcb36b 100644
--- a/libass/ass_shaper.c
+++ b/libass/ass_shaper.c
@@ -1020,6 +1020,11 @@ FriBidiStrIndex *ass_shaper_reorder(ASS_Shaper *shaper, TextInfo *text_info)
     return shaper->cmap;
 }
 
+FriBidiStrIndex *ass_shaper_get_reorder_map(ASS_Shaper *shaper)
+{
+    return shaper->cmap;
+}
+
 /**
  * \brief Resolve a Windows font charset number to a suitable base
  * direction. Generally, use LTR for compatibility with VSFilter. The