Apple GPUs do not support rendering/writing to an attachment and then reading
from that attachment within a single Metal renderpass. On Apple Silicon, restart
the Metal renderpass if an input attachment is cleared inside renderpass.
Don't clear render area when restarting Metal renderpass, as it should not occur,
and itself causes a recursive loop restarting the renderpass as a result.
Add MVKCommandUse::kMVKCommandUseRestartSubpass.
MVKCommandEncoder::beginMetalRenderPass() pass MVKCommandUse to help determine
Metal renderpass attachment load and clearing options.
8a7e20d348