Only reallocate MegaBuffer on usage

Certain submissions might not utilize megabuffering but reserve a `MegaBuffer` regardless, this is not optimal since it can inflate the allocations and waste memory. This commit addresses the issue by eliding the allocation given the current submission doesn't utilize them.

app/src/main/cpp/skyline/gpu/buffer_manager.cpp

@@ -119,6 +119,14 @@ namespace skyline::gpu {
         return *this;
     }
 
+    bool MegaBuffer::WasUsed() {
+        return freeRegion != slot->backing.subspan(PAGE_SIZE);
+    }
+
+    void MegaBuffer::ReplaceCycle(const std::shared_ptr<FenceCycle> &cycle) {
+        slot->cycle = cycle;
+    }
+
     void MegaBuffer::Reset() {
         freeRegion = slot->backing.subspan(PAGE_SIZE);
     }

app/src/main/cpp/skyline/gpu/buffer_manager.h

@@ -91,6 +91,17 @@ namespace skyline::gpu {
 
         MegaBuffer &operator=(MegaBuffer &&other);
 
+        /**
+         * @return If any allocations into the megabuffer were done at the time of the call
+         */
+        bool WasUsed();
+
+        /**
+         * @brief Replaces the cycle associated with the underlying megabuffer with the supplied cycle
+         * @note The megabuffer must **NOT** have any dependencies that aren't conveyed by the supplied cycle
+         */
+        void ReplaceCycle(const std::shared_ptr<FenceCycle> &cycle);
+
         /**
          * @brief Resets the free region of the megabuffer to its initial state, data is left intact but may be overwritten
          */

app/src/main/cpp/skyline/gpu/interconnect/command_executor.cpp

@@ -321,7 +321,10 @@ namespace skyline::gpu::interconnect {
             SubmitInternal();
             activeCommandBuffer = gpu.scheduler.AllocateCommandBuffer();
             cycle = activeCommandBuffer.GetFenceCycle();
-            megaBuffer = gpu.buffer.AcquireMegaBuffer(cycle);
+            if (megaBuffer.WasUsed())
+                megaBuffer = gpu.buffer.AcquireMegaBuffer(cycle);
+            else
+                megaBuffer.ReplaceCycle(cycle);
         }
         ResetInternal();
     }