[Mammoth] Get rid of last uses of MappedMemoryRegion.

sys/teton/framebuffer/framebuffer.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <mammoth/memory_region.h>
 #include <yellowstone/yellowstone.yunq.h>
 
 class Framebuffer {
@@ -12,5 +13,7 @@ class Framebuffer {
   // FIXME: Implement Yunq copy or move so we
   // don't have to store a reference here.
   const FramebufferInfo& fb_info_;
+
+  OwnedMemoryRegion fb_memory_;
   uint32_t* fb_;
 };