REG: Qt now using separate index and vertex buffer in Qt Quick

In 2c2e7f27426cb4381f2dcaa36affe55719619753, we removed the shared index/vertex buffer from the Qt Quick scenegraph because of missing support on WebGL. This causes a regression in performance and should be reverted.

Ideally it should be detected at runtime, but if that is not feasible, we should disable WebGL by default in the builds and then only use separate buffers when we are explicitly building for WebGL support.