Log: Switch to enum class

Need to change the channel to a bitset too.. the string lookups are
horribly slow, and conflict when one is a prefix of another.

src/util/metal_stream_buffer.mm

@@ -8,7 +8,7 @@
 #include "common/assert.h"
 #include "common/log.h"
 
-Log_SetChannel(MetalDevice);
+LOG_CHANNEL(MetalDevice);
 
 MetalStreamBuffer::MetalStreamBuffer() = default;