Determine if Metal needs an off-screen surface #43789
Description
On OpenGL, we use an off-screen rendering surface to improve the performance of BackdropFilters. However, this seems to come at a penalty of extra CPU/GPU usage overall (#31865 (comment) and following comments).
Metal doesn't (yet?) use an off-screen surface. We should determine if using one on the Metal backend causes the same increase in CPU/GPU usage, and what impact (if any) not using it has on backdrop filters.