Tracing "Timeline" requires an OpenGL renderer for QtQuick

The Timeline has multiple custom shaders which only work with OpenGL. When OpenGL is unavailable, for example when using the software renderer, we switch the whole timeline off. Until now this was somewhat acceptable because OpenGL seemed to be the way "forward" in graphics software.

Recently Apple decided to deprecate OpenGL on all of their platforms. In order to keep the Timeline available on macOS we will have to be compatible with other QtQuick renderers eventually. macOS apparently supports Vulkan and Metal. In general OpenGL seems to not be the way "forward" anymore.