ddr: tweaks to mdxf i/o (#464)

## Link to GitHub Issue, if one exists
#452

## Description of change
* Split the MDXF I/O option into three: auto, thread, and backfill.
`auto` is the same as what `thread` used to be before this change (based
on I/O poll rate), but `thread` now always forces thread poll mode.
`auto` is the default (behavior wise, same as before)
* For the poll rate measurement, start polling after 20s mark to avoid
frame drops during game boot
* Minor code clean up

## Testing
manual test in windowed + full screen

src/spice2x/acio/mdxf/mdxf.h

@@ -3,11 +3,17 @@
 #include "../module.h"
 
 namespace acio {
-    
-    typedef enum {
-        THREAD_MODE = 0,
-        BACKFILL_MODE = 1
-    } MDXFBufferFillMode;
+
+    enum class MDXFBufferFillMode {
+        // backfill mode, but if <120Hz, enable poll thread
+        AUTO_MODE,
+
+        // forces poll thread
+        THREAD_MODE,
+
+        // forces backfill mode (no poll thread)
+        BACKFILL_MODE
+    };
 
     extern MDXFBufferFillMode MDXF_BUFFER_FILL_MODE;