refactor vao object to use acutal OpenGL IDs rather than object pointers

src/geometry/vao.cpp

@@ -0,0 +1,46 @@
+#include <mbgl/geometry/vao.hpp>
+
+namespace mbgl {
+
+#if GL_ARB_vertex_array_object
+
+VertexArrayObject::~VertexArrayObject() {
+    if (vao) {
+        glDeleteVertexArrays(1, &vao);
+    }
+}
+
+void VertexArrayObject::bindVertexArrayObject() {
+    if (!vao) {
+        glGenVertexArrays(1, &vao);
+    }
+    glBindVertexArray(vao);
+}
+
+void VertexArrayObject::verifyBinding(Shader &shader, GLuint vertexBuffer, GLuint elementsBuffer,
+                                      char *offset) {
+    if (bound_shader != shader.getID()) {
+        throw std::runtime_error(std::string("trying to rebind VAO to another shader from " +
+                                             std::to_string(bound_shader) + "(" + bound_shader_name + ") to " +
+                                             std::to_string(shader.getID()) + "(" + shader.name + ")" ));
+    } else if (bound_offset != offset) {
+        throw std::runtime_error("trying to bind VAO to another offset");
+    } else if (bound_vertex_buffer != vertexBuffer) {
+        throw std::runtime_error("trying to bind VAO to another vertex buffer");
+    } else if (bound_elements_buffer != elementsBuffer) {
+        throw std::runtime_error("trying to bind VAO to another elements buffer");
+    }
+}
+
+void VertexArrayObject::storeBinding(Shader &shader, GLuint vertexBuffer, GLuint elementsBuffer,
+                                     char *offset) {
+    bound_shader = shader.getID();
+    bound_shader_name = shader.name;
+    bound_offset = offset;
+    bound_vertex_buffer = vertexBuffer;
+    bound_elements_buffer = elementsBuffer;
+}
+
+#endif
+
+}

src/geometry/vertex_buffer.cpp

@@ -26,3 +26,7 @@ void VertexBuffer::bind() {
         glBindBuffer(GL_ARRAY_BUFFER, buffer);
     }
 }
+
+uint32_t VertexBuffer::getID() const {
+    return buffer;
+}