[Impeller] Fix glyph atlas uploads and renders (flutter#37691)

Fixes flutter/flutter#115461

Author: iskakaushik

impeller/renderer/backend/vulkan/allocator_vk.cc

@@ -12,6 +12,7 @@
 #include "impeller/renderer/backend/vulkan/device_buffer_vk.h"
 #include "impeller/renderer/backend/vulkan/formats_vk.h"
 #include "impeller/renderer/backend/vulkan/texture_vk.h"
+#include "impeller/renderer/formats.h"
 
 namespace impeller {
 
@@ -146,6 +147,15 @@ std::shared_ptr<Texture> AllocatorVK::OnCreateTexture(
   view_create_info.subresourceRange.levelCount = image_create_info.mipLevels;
   view_create_info.subresourceRange.layerCount = image_create_info.arrayLayers;
 
+  // Vulkan does not have an image format that is equivalent to
+  // `MTLPixelFormatA8Unorm`, so we use `R8Unorm` instead. Given that the
+  // shaders expect that alpha channel to be set in the cases, we swizzle.
+  // See: https://github.com/flutter/flutter/issues/115461 for more details.
+  if (desc.format == PixelFormat::kA8UNormInt) {
+    view_create_info.components.a = vk::ComponentSwizzle::eR;
+    view_create_info.components.r = vk::ComponentSwizzle::eA;
+  }
+
   auto img_view_res = device_.createImageView(view_create_info);
   if (img_view_res.result != vk::Result::eSuccess) {
     VALIDATION_LOG << "Unable to create an image view: "

impeller/renderer/backend/vulkan/formats_vk.h

@@ -139,7 +139,7 @@ constexpr vk::Format ToVKImageFormat(PixelFormat format) {
     case PixelFormat::kUnknown:
       return vk::Format::eUndefined;
     case PixelFormat::kA8UNormInt:
-      return vk::Format::eA8B8G8R8UnormPack32;
+      return vk::Format::eR8Unorm;
     case PixelFormat::kR8G8B8A8UNormInt:
       return vk::Format::eR8G8B8A8Unorm;
     case PixelFormat::kR8G8B8A8UNormIntSRGB:
@@ -164,9 +164,6 @@ constexpr PixelFormat ToPixelFormat(vk::Format format) {
     case vk::Format::eUndefined:
       return PixelFormat::kUnknown;
 
-    case vk::Format::eA8B8G8R8UnormPack32:
-      return PixelFormat::kA8UNormInt;
-
     case vk::Format::eR8G8B8A8Unorm:
       return PixelFormat::kR8G8B8A8UNormInt;