fix hyrule magic decompression memory leak, misc housekeeping

src/app/gui/canvas.cc

@@ -173,6 +173,8 @@ bool Canvas::DrawTilePainter(const Bitmap &bitmap, int size, float scale) {
     painter_pos.y =
         std::floor((double)mouse_pos.y / (size * scale)) * (size * scale);
 
+    mouse_pos_in_canvas_ = painter_pos;
+
     auto painter_pos_end =
         ImVec2(painter_pos.x + (size * scale), painter_pos.y + (size * scale));
     points_.push_back(painter_pos);

src/app/gui/canvas.h

@@ -163,6 +163,8 @@ class Canvas {
   bool select_rect_active() const { return select_rect_active_; }
   auto selected_points() const { return selected_points_; }
 
+  auto hover_mouse_pos() const { return mouse_pos_in_canvas_; }
+
  private:
   bool draggable_ = false;
   bool enable_grid_ = true;