[Impeller] match Skia's old VMA default block size. (#56368)

The unspecified default is 256 MiB which is ... big. 4Mb value is the default that Skia used to use, and results in less memory usage at least in local testing.

benchmarks will determine if this has a positive or negative impact on performance.

https://github.com/flutter/flutter/issues/157497

impeller/renderer/backend/vulkan/allocator_vk.cc

diff --git a/impeller/renderer/backend/vulkan/allocator_vk.cc b/impeller/renderer/backend/vulkan/allocator_vk.cc
index ae760a6..f66bbab 100644
--- a/impeller/renderer/backend/vulkan/allocator_vk.cc
+++ b/impeller/renderer/backend/vulkan/allocator_vk.cc
@@ -84,6 +84,7 @@
   VmaPoolCreateInfo pool_create_info = {};
   pool_create_info.memoryTypeIndex = memTypeIndex;
   pool_create_info.flags = VMA_POOL_CREATE_IGNORE_BUFFER_IMAGE_GRANULARITY_BIT;
+  pool_create_info.minBlockCount = 1;
 
   VmaPool pool = {};
   result = vk::Result{::vmaCreatePool(allocator, &pool_create_info, &pool)};
@@ -144,6 +145,8 @@
   allocator_info.physicalDevice = physical_device;
   allocator_info.device = device_holder->GetDevice();
   allocator_info.instance = instance;
+  // 4 MB, matching the default used by Skia Vulkan.
+  allocator_info.preferredLargeHeapBlockSize = 4 * 1024 * 1024;
   allocator_info.pVulkanFunctions = &proc_table;
 
   VmaAllocator allocator = {};