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/postprocessing.cpp

@@ -26,7 +26,7 @@
 #include "common/timer.h"
 #include "fmt/format.h"
 
-Log_SetChannel(PostProcessing);
+LOG_CHANNEL(PostProcessing);
 
 // TODO: ProgressCallbacks for shader compiling, it can be a bit slow.
 // TODO: buffer width/height is wrong on resize, need to change it somehow.