Fix drawable segment VAO leak (#1212)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

include/mbgl/gfx/drawable_impl.hpp

@@ -10,6 +10,8 @@ struct Drawable::DrawSegment {
         : mode(mode_),
           segment(std::move(segment_)) {}
 
+    virtual ~DrawSegment() = default;
+
     const gfx::DrawMode& getMode() const { return mode; }
 
     Segment<void>& getSegment() { return segment; }

src/mbgl/gl/drawable_gl_impl.hpp

@@ -53,6 +53,8 @@ struct DrawableGL::DrawSegmentGL final : public gfx::Drawable::DrawSegment {
         : gfx::Drawable::DrawSegment(mode_, std::move(segment_)),
           vertexArray(std::move(vertexArray_)) {}
 
+    ~DrawSegmentGL() override = default;
+
     const VertexArray& getVertexArray() const { return vertexArray; }
     void setVertexArray(VertexArray&& value) { vertexArray = std::move(value); }