BufferCache: Use a CircularBuffer as the transient buffer.

src/xenia/ui/vulkan/circular_buffer.cc

@@ -134,12 +134,13 @@ CircularBuffer::Allocation* CircularBuffer::Acquire(
   }
 
   VkDeviceSize aligned_length = xe::round_up(length, alignment_);
+  assert_true(write_head_ % alignment_ == 0);
   if (allocations_.empty()) {
     // Entire buffer available.
     assert(read_head_ == write_head_);
     assert(capacity_ > aligned_length);
 
-    write_head_ = length;
+    write_head_ = aligned_length;
 
     auto alloc = new Allocation();
     alloc->host_ptr = host_base_ + 0;

src/xenia/ui/vulkan/circular_buffer.h

@@ -46,6 +46,7 @@ class CircularBuffer {
                   VkDeviceSize alignment = 256);
   void Shutdown();
 
+  VkDeviceSize alignment() const { return alignment_; }
   VkDeviceSize capacity() const { return capacity_; }
   VkBuffer gpu_buffer() const { return gpu_buffer_; }
   VkDeviceMemory gpu_memory() const { return gpu_memory_; }