Don't crash if VAO extensions are not available

Fixes #1585

src/mbgl/geometry/vao.cpp

@@ -31,8 +31,9 @@ static gl::ExtensionFunction<
         {"GL_APPLE_vertex_array_object", "glGenVertexArraysAPPLE"}
     });
 
-void VertexArrayObject::Bind(GLuint array) {
-    MBGL_CHECK_ERROR(BindVertexArray(array));
+void VertexArrayObject::Unbind() {
+    if (!BindVertexArray) return;
+    MBGL_CHECK_ERROR(BindVertexArray(0));
 }
 
 void VertexArrayObject::Delete(GLsizei n, const GLuint* arrays) {

src/mbgl/geometry/vao.hpp

@@ -11,7 +11,7 @@ namespace mbgl {
 
 class VertexArrayObject : public util::noncopyable {
 public:
-    static void Bind(GLuint array);
+    static void Unbind();
     static void Delete(GLsizei n, const GLuint* arrays);
 
     VertexArrayObject();

src/mbgl/renderer/painter.cpp

@@ -263,7 +263,7 @@ void Painter::render(const Style& style, TransformState state_, TimePoint time)
         const gl::debugging::group _("cleanup");
 
         MBGL_CHECK_ERROR(glBindTexture(GL_TEXTURE_2D, 0));
-        MBGL_CHECK_ERROR(VertexArrayObject::Bind(0));
+        MBGL_CHECK_ERROR(VertexArrayObject::Unbind());
     }
 }