[core] Fix debug rendering

* Off-by-one in DebugBucket indexing
* Must unbind VAO before binding index buffer in Context::createIndexBuffer

src/mbgl/gl/context.cpp

@@ -105,6 +105,7 @@ UniqueBuffer Context::createIndexBuffer(const void* data, std::size_t size) {
     BufferID id = 0;
     MBGL_CHECK_ERROR(glGenBuffers(1, &id));
     UniqueBuffer result { std::move(id), { this } };
+    vertexArrayObject = 0;
     elementBuffer = result;
     MBGL_CHECK_ERROR(glBufferData(GL_ELEMENT_ARRAY_BUFFER, size, data, GL_STATIC_DRAW));
     return result;

src/mbgl/renderer/debug_bucket.cpp

@@ -46,8 +46,8 @@ DebugBucket::DebugBucket(const OverscaledTileID& id,
                     vertices.emplace_back(FillAttributes::vertex(p));
 
                     if (prev) {
-                        indices.emplace_back(vertices.vertexSize() - 1,
-                                             vertices.vertexSize());
+                        indices.emplace_back(vertices.vertexSize() - 2,
+                                             vertices.vertexSize() - 1);
                     }
 
                     prev = p;