Restructure how buffers are set up (#77)

src/buffers/gfxbuffers.c

@@ -0,0 +1,8 @@
+#include <ultra64.h>
+#include <global.h>
+
+u8 gGfxSPTaskYieldBuffer[OS_YIELD_DATA_SIZE];
+
+u8 gGfxSPTaskStack[0x400];
+
+GfxPool gGfxPools[2];

src/buffers/heaps.c

@@ -0,0 +1,6 @@
+#include <ultra64.h>
+#include <global.h>
+
+u8 gAudioHeap[0x138000];
+
+u32 gSystemHeap[UNK_SIZE];