fix(freetpye): fix the problem of incomplete font drawing when setting with italic (#5807)

src/libs/freetype/lv_freetype.c

@@ -20,6 +20,9 @@
 #define ft_ctx LV_GLOBAL_DEFAULT()->ft_context
 #define LV_FREETYPE_OUTLINE_REF_SIZE_DEF 128
 
+/**< This value is from the FreeType's function `FT_GlyphSlot_Oblique` in `ftsynth.c` */
+#define LV_FREETYPE_OBLIQUE_SLANT_DEF 0x5800
+
 #if LV_FREETYPE_CACHE_FT_GLYPH_CNT <= 0
     #error "LV_FREETYPE_CACHE_FT_GLYPH_CNT must be greater than 0"
 #endif
@@ -244,12 +247,17 @@ void lv_freetype_italic_transform(FT_Face face)
     LV_ASSERT_NULL(face);
     FT_Matrix matrix;
     matrix.xx = FT_INT_TO_F16DOT16(1);
-    matrix.xy = 0x5800;
+    matrix.xy = LV_FREETYPE_OBLIQUE_SLANT_DEF;
     matrix.yx = 0;
     matrix.yy = FT_INT_TO_F16DOT16(1);
     FT_Set_Transform(face, &matrix, NULL);
 }
 
+int32_t lv_freetype_italic_transform_on_pos(lv_point_t point)
+{
+    return point.x + FT_F16DOT16_TO_INT(point.y * LV_FREETYPE_OBLIQUE_SLANT_DEF);
+}
+
 const char * lv_freetype_get_pathname(FTC_FaceID face_id)
 {
     LV_ASSERT_NULL(face_id);

src/libs/freetype/lv_freetype_glyph.c

@@ -150,6 +150,13 @@ static bool freetype_glyph_create_cb(lv_freetype_glyph_cache_data_t * data, void
         dsc_out->ofs_y = FT_F26DOT6_TO_INT(glyph->metrics.horiBearingY -
                                            glyph->metrics.height);          /*Y offset of the bitmap measured from the as line*/
         dsc_out->format = LV_FONT_GLYPH_FORMAT_VECTOR;
+
+        /*Transform the glyph to italic if required*/
+        if(dsc->style & LV_FREETYPE_FONT_STYLE_ITALIC) {
+            dsc_out->box_w = lv_freetype_italic_transform_on_pos((lv_point_t) {
+                dsc_out->box_w, dsc_out->box_h
+            });
+        }
     }
     else if(dsc->render_mode == LV_FREETYPE_FONT_RENDER_MODE_BITMAP) {
         FT_Bitmap * glyph_bitmap = &face->glyph->bitmap;

src/libs/freetype/lv_freetype_private.h

@@ -121,6 +121,7 @@ lv_freetype_context_t * lv_freetype_get_context(void);
 FT_Size lv_freetype_lookup_size(const lv_freetype_font_dsc_t * dsc);
 
 void lv_freetype_italic_transform(FT_Face face);
+int32_t lv_freetype_italic_transform_on_pos(lv_point_t point);
 
 const char * lv_freetype_get_pathname(FTC_FaceID face_id);