[Impeller] Conditionally use A8 or R8 format glyph atlas based on capabilities. (#50534)

R8 texture format is more widely supported than A8, which we emulate on the vulkan backend with a swizzle. Rather than adding more capability or swizzling checks lets just switch to R8 which is fairly trivial. (I think some desktop gl does not support A8 too, but not sure).

Add Capabilities getDefaultAtlasFormat. Set this to R8 in Vulkan an non-ES GL. A8 Elsewhere.
https://dart.googlesource.com/external/github.com/flutter/engine/+/fa53031f8d17392fffc6e988b6a51344c03feda7

DEPS

diff --git a/DEPS b/DEPS
index d46fb57..c7dfd1c 100644
--- a/DEPS
+++ b/DEPS
@@ -277,7 +277,7 @@
 deps = {
   'engine/src': 'https://github.com/flutter/buildroot.git' + '@' + '5413806166ee3b5198b9dd1c1f684d266ad9850c',
 
-  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + 'a9081cce1f0dd730577a36ee1ca6d7af5cdc5a9b',
+  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + 'fa53031f8d17392fffc6e988b6a51344c03feda7',
 
   'engine/src/third_party/dart': Var('dart_git') + '/sdk' + '@' + '885126e51bf2d0c612a42ba55395ac4f4d9f7b42',
 

commits.json

diff --git a/commits.json b/commits.json
index af235af..cecf8cb 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "engine/src/flutter":"a9081cce1f0dd730577a36ee1ca6d7af5cdc5a9b",
+  "engine/src/flutter":"fa53031f8d17392fffc6e988b6a51344c03feda7",
   "engine/src/third_party/dart":"885126e51bf2d0c612a42ba55395ac4f4d9f7b42",
   "flutter":"c61dc2a586dda907fb6f4e99647d4bc6f29ddf8a"
 }
\ No newline at end of file