[Impeller] new blur: round downsample to power of two (#50245)

This makes the downsampling step of the new blur round to the nearest power of 2.  This makes the changes in downsampling less frequent and the output of downsampling hypothetically higher quality since downsampling by a power of 2 is easier.

issue: https://github.com/flutter/flutter/issues/141510

## before
https://github.com/flutter/engine/assets/30870216/73e78f02-1346-4dde-ad6e-8aaa7c910bac

## after
https://github.com/flutter/engine/assets/30870216/b4153ada-6a82-4d0a-a4c2-158134c7b74f

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
https://dart.googlesource.com/external/github.com/flutter/engine/+/9beb7e82e081d963bdf7d5342c0f6a16f4edc2b6

DEPS

diff --git a/DEPS b/DEPS
index 670f1b5..e8f8b48 100644
--- a/DEPS
+++ b/DEPS
@@ -277,7 +277,7 @@
 deps = {
   'engine/src': 'https://github.com/flutter/buildroot.git' + '@' + '52689fe8955a9a4916a38f0b8ae5ea3e217037f4',
 
-  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + 'fa77602e788085bcf542cf9ed30674b641284e55',
+  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + '9beb7e82e081d963bdf7d5342c0f6a16f4edc2b6',
 
   'engine/src/third_party/dart': Var('dart_git') + '/sdk' + '@' + '581686b8fbd2466f0df794a6071a07446e2788e5',
 

commits.json

diff --git a/commits.json b/commits.json
index e65611c..8bc5704 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "engine/src/flutter":"fa77602e788085bcf542cf9ed30674b641284e55",
+  "engine/src/flutter":"9beb7e82e081d963bdf7d5342c0f6a16f4edc2b6",
   "engine/src/third_party/dart":"581686b8fbd2466f0df794a6071a07446e2788e5",
   "flutter":"cd6ed39550d8794f50cb1bcda2c59b9ceb0d3ccb"
 }
\ No newline at end of file