[COLRv1] Push font transform trickery to hb-cairo

This simplifies the renderers but pushes the burdon to clients
of the paint API. I am ambivalent about this change. All paint
clients need to be updated.

This reduces one vector of allocations from hb-fontations paint
implementation however.

src/OT/Color/COLR/COLR.hh

@@ -938,13 +938,9 @@ struct PaintGlyph
   void paint_glyph (hb_paint_context_t *c) const
   {
     TRACE_PAINT (this);
-    c->funcs->push_inverse_font_transform (c->data, c->font);
     c->funcs->push_clip_glyph (c->data, gid, c->font);
-    c->funcs->push_font_transform (c->data, c->font);
     c->recurse (this+paint);
-    c->funcs->pop_transform (c->data);
     c->funcs->pop_clip (c->data);
-    c->funcs->pop_transform (c->data);
   }
 
   HBUINT8		format; /* format = 10 */
@@ -2819,13 +2815,8 @@ void PaintColrGlyph::paint_glyph (hb_paint_context_t *c) const
   if (unlikely (!node.visit (gid)))
     return;
 
-  c->funcs->push_inverse_font_transform (c->data, c->font);
   if (c->funcs->color_glyph (c->data, gid, c->font))
-  {
-    c->funcs->pop_transform (c->data);
     return;
-  }
-  c->funcs->pop_transform (c->data);
 
   const COLR *colr_table = c->get_colr_table ();
   const Paint *paint = colr_table->get_base_glyph_paint (gid);

src/fontations/lib.rs

@@ -416,7 +416,6 @@ struct HbColorPainter<'a> {
     color_records: &'a [ColorRecord],
     foreground: hb_color_t,
     composite_mode: Vec<CompositeMode>,
-    clip_transform_stack: Vec<bool>,
 }
 
 impl HbColorPainter<'_> {
@@ -546,20 +545,16 @@ impl ColorPainter for HbColorPainter<'_> {
     }
     fn push_clip_glyph(&mut self, glyph: GlyphId) {
         let gid = u32::from(glyph);
-        self.clip_transform_stack.push(true);
         unsafe {
-            hb_paint_push_inverse_font_transform(self.paint_funcs, self.paint_data, self.font);
             hb_paint_push_clip_glyph(
                 self.paint_funcs,
                 self.paint_data,
                 gid as hb_codepoint_t,
                 self.font,
             );
-            hb_paint_push_font_transform(self.paint_funcs, self.paint_data, self.font);
         }
     }
     fn push_clip_box(&mut self, bbox: BoundingBox) {
-        self.clip_transform_stack.push(false);
         unsafe {
             hb_paint_push_clip_rectangle(
                 self.paint_funcs,
@@ -572,16 +567,9 @@ impl ColorPainter for HbColorPainter<'_> {
         }
     }
     fn pop_clip(&mut self) {
-        let pop_transforms = self.clip_transform_stack.pop().unwrap_or(false);
-        if pop_transforms {
-            self.pop_transform();
-        }
         unsafe {
             hb_paint_pop_clip(self.paint_funcs, self.paint_data);
         }
-        if pop_transforms {
-            self.pop_transform();
-        }
     }
     fn fill(&mut self, brush: Brush) {
         match brush {
@@ -767,7 +755,6 @@ extern "C" fn _hb_fontations_paint_glyph(
         color_records,
         foreground,
         composite_mode: Vec::new(),
-        clip_transform_stack: Vec::new(),
     };
     unsafe {
         hb_paint_push_font_transform(paint_funcs, paint_data, font);

src/hb-cairo.cc

@@ -193,7 +193,7 @@ hb_cairo_paint_color_glyph (hb_paint_funcs_t *pfuncs HB_UNUSED,
 }
 
 static void
-hb_cairo_push_clip_glyph (hb_paint_funcs_t *pfuncs HB_UNUSED,
+hb_cairo_push_clip_glyph (hb_paint_funcs_t *pfuncs,
 			  void *paint_data,
 			  hb_codepoint_t glyph,
 			  hb_font_t *font,
@@ -204,7 +204,11 @@ hb_cairo_push_clip_glyph (hb_paint_funcs_t *pfuncs HB_UNUSED,
 
   cairo_save (cr);
   cairo_new_path (cr);
+
+  hb_paint_push_inverse_font_transform (pfuncs, paint_data, font);
   hb_font_draw_glyph (font, glyph, hb_cairo_draw_get_funcs (), cr);
+  hb_paint_push_font_transform (pfuncs, paint_data, font);
+
   cairo_close_path (cr);
   cairo_clip (cr);
 }

src/hb-ft-colr.hh

@@ -316,15 +316,11 @@ _hb_ft_paint (hb_ft_paint_context_t *c,
     break;
     case FT_COLR_PAINTFORMAT_GLYPH:
     {
-      c->funcs->push_inverse_font_transform (c->data, c->font);
       c->ft_font->lock.unlock ();
       c->funcs->push_clip_glyph (c->data, paint.u.glyph.glyphID, c->font);
       c->ft_font->lock.lock ();
-      c->funcs->push_font_transform (c->data, c->font);
       c->recurse (paint.u.glyph.paint);
-      c->funcs->pop_transform (c->data);
       c->funcs->pop_clip (c->data);
-      c->funcs->pop_transform (c->data);
     }
     break;
     case FT_COLR_PAINTFORMAT_COLR_GLYPH:
@@ -335,16 +331,13 @@ _hb_ft_paint (hb_ft_paint_context_t *c,
       if (unlikely (!node.visit (gid)))
 	return;
 
-      c->funcs->push_inverse_font_transform (c->data, c->font);
       c->ft_font->lock.unlock ();
       if (c->funcs->color_glyph (c->data, gid, c->font))
       {
 	c->ft_font->lock.lock ();
-	c->funcs->pop_transform (c->data);
 	return;
       }
       c->ft_font->lock.lock ();
-      c->funcs->pop_transform (c->data);
 
       FT_OpaquePaint other_paint = {0};
       if (FT_Get_Color_Glyph_Paint (ft_face, gid,