Fixed StyleBoxFlat antialiasing to be applied only on corners

After multiple changes to the transparent drawing used for AA, it ended up being consistently applied all around the borders inside and outside, which is not the intended behavior. This change makes sure border flat sides are solid instead of blurry.

Fixes godotengine#35279

scene/resources/style_box.cpp

@@ -719,7 +719,7 @@ void StyleBoxFlat::draw(RID p_canvas_item, const Rect2 &p_rect) const {
 
  bool rounded_corners = (corner_radius[0] > 0) || (corner_radius[1] > 0) || (corner_radius[2] > 0) || (corner_radius[3] > 0);
  bool aa_on = rounded_corners && anti_aliased;
- float aa_size_grow = 0.5 * ((float)aa_size + 1.0);
+ float aa_size_grow = 0.5 * ((aa_size + 1) / 2);
 
  bool blend_on = blend_border && draw_border;
 
@@ -789,6 +789,8 @@ void StyleBoxFlat::draw(RID p_canvas_item, const Rect2 &p_rect) const {
  }
 
  if (aa_on) {
+ float aa_size_fill = 0.5 * ((float)aa_size + 1.0);
+
  int aa_border_width[4];
  int aa_fill_width[4];
  if (draw_border) {
@@ -798,13 +800,13 @@ void StyleBoxFlat::draw(RID p_canvas_item, const Rect2 &p_rect) const {
  aa_fill_width[i] = 0;
  } else {
  aa_border_width[i] = 0;
- aa_fill_width[i] = aa_size_grow;
+ aa_fill_width[i] = aa_size_fill;
  }
  }
  } else {
  for (int i = 0; i < 4; i++) {
  aa_border_width[i] = 0;
- aa_fill_width[i] = aa_size_grow;
+ aa_fill_width[i] = aa_size_fill;
  }
  }