Settings: Change default GPU FIFO size to 16

While this has the potential to be slower, there seems to be quite a few
games which break with the larger FIFO. In some cases, they wait for the
DMA interrupt, then busy wait for the GPU to go idle again. If the FIFO
is larger, this wait time till be longer, breaking things.

src/core/settings.h

@@ -120,7 +120,7 @@ struct Settings
   {
     DEFAULT_DMA_MAX_SLICE_TICKS = 1000,
     DEFAULT_DMA_HALT_TICKS = 100,
-    DEFAULT_GPU_FIFO_SIZE = 128,
+    DEFAULT_GPU_FIFO_SIZE = 16,
     DEFAULT_GPU_MAX_RUN_AHEAD = 128
   };